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paleoanthropology, genetics and evolution

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stature

  • Stature estimates for Sima de los Huesos

    Tue, 2012-01-10 00:44 -- John Hawks

    José-Miguel Carretero and colleagues [1] report on the lengths of long bones from Sima de los Huesos, Spain. I've long been hoping this research would come out, because we've gotten interested in the pattern of body size as an aspect of evolution in early Homo.

    Sima de los Huesos is the single largest sample of fossil Homo, and Carretero and colleagues include 27 mostly complete long bones in their sample. That's around a dozen fewer than the entire sample of Neandertal long bones. This one site has more long bones than the rest of the Early and Middle Pleistocene combined.

    Here are the tibiae, for example:

    Tibiae from Sima de los Huesos

    Complete tibiae from Sima de los Huesos, from Carretero et al. [1], figure 2.

    The paper shows that the Sima hominins averaged a bit taller than Neandertals for most of the long bones.

    That conclusion isn't quite as simple as it might look, because the sample of male Neandertal femora actually average 3 mm longer than the Sima de los Huesos femora. Both samples have more than double the number of males as females, so the male comparison draws on a much larger sample size. The Neandertal male femoral sample is biased a bit high by the inclusion of both left and right femora from Amud, the tallest of the Neandertal skeletons. The tibia sample gives a substantially shorter stature for Neandertal males, both because Amud isn't there, and because the limb proportions of Neandertals have short tibiae relative to their femora.

    That's the problem of using stature estimates instead of simple bone lengths: Nothing's simple. Fossil samples impose some limits on the kind of analyses we can undertake. Carretero and colleagues address stature both because of its biological relevance and because estimating stature is the most reasonable way we can incorporate different long bones into a single size comparison. But considering stature introduces some problems of estimation. We can't be sure how many individuals are represented by the long bones. We can determine a minimum: Six right tibiae came from a minimum of six bodies, for example. But if two arm bones and a leg bone all came from the same skeleton, that individual will be represented three times within this sample, and we don't have a way to exclude that possibility. Worse, estimating stature requires a regression drawn from some population, but that population may have different proportions than the fossils. In this case, Neandertals and the Sima de los Huesos samples probably have different crural indices, the ratio of the length of the tibia to the length of the femur. So statures estimated from these bone lengths based on some recent human population will have systematic biases due to the different proportions in the fossil populations.

    Carretero and colleagues note that most of the bones (humerus, radius, tibia) have shorter average statures in the Neandertal sample compared to the Sima de los Huesos sample. The femora and ulnae are longer in the male Neandertals. All the bones that can be compared are shorter in the female Neandertals than the female Sima de los Huesos individuals. It's probably a good bet that the Sima people were a bit taller than Neandertals. Still, the tall West Asian Amud skeleton points to the possibility of variation among Neandertals from different regions.

    The differences between Neandertals and the Sima de los Huesos sample are quite small compared to the much taller statures attributed to modern humans from West Asia (Skhul and Qafzeh). These skeletons are more ancient than most of the Neandertal sample, but at 100,000 years old, much later than the other skeletal samples included in the paper including Sima de los Huesos. The authors make a strong point of this, suggesting that tall stature is a fundamentally new feature of the evolution of modern humans (which they equate with "early H. sapiens"):

    As we have shown here, ‘medium height’ and ‘above-medium height’ people seem to characterize the primitive Homo biotype, while a ‘very tall’ body characterizes the derived biotype. The heights proposed for all fossil human species, except early H. sapiens, seem to average around 165–170 cm, although tall individuals exist within all samples (e.g., Amud 1, Kabwe and Jinniushan). It is only the first H. sapiens that are consistently and dramatically taller. Therefore, the evolution of stature (and perhaps also body size and shape) in humans seems to have been characterized by a long period of stasis during which the primitive body plan shared by the different Homo species varied rather little in stature throughout the Pleistocene, until the rapid appearance 200 ka of a new species with a new biotype, the ‘light’ H. sapiens.

    The paper's broad assertion is that Early and Middle Pleistocene humans everywhere in the world shared the same basic body plan, with stature around 165-170 cm (for males) and relatively broad pelves. The reference to modern humans as "light" concerns the relatively narrower pelvis of recent humans.

    I have no disagreement about the issue of pelvic breadth, although it deserves a separate review. But the stature of the Skhul-Qafzeh sample is neither very extreme nor is it typical of other Late Pleistocene or Holocene modern human samples. I will reprint a quote from my 2007 post about the statures of the Dmanisi hominins ("News flash: Dmanisi hominids were not short"):

    Pretty and colleagues (1998) studied an archaeological sample of Aboriginal Australians from the Murray River region. Using stature estimation methods for the tibia, femur and humerus, they found that males in their sample (n=55) had an average stature of 166 cm and females (n=40) an average of around 153 cm. Wells (1952) reported a mean for !Khu (Northern Bushmen) males of 158 cm and females of 148 cm, both with standard deviations around 5 cm. Ruff (2000) puts the average stature of males at Pecos Pueblo at 161.2 cm with a range from 155 to 168 cm. In the KNM-WT 15000 monograph, Ruff and Walker (1993) report the average stature of African population samples, excluding Pygmies, as 162.3 cm. And although it is common knowledge that the Early Upper Paleolithic people of Europe were tall, the average male stature in the Late Upper Paleolithic was around 166 cm, and the average female stature around 153 cm (Formicola and Giannecchini 1999) -- virtually the same as Australians.

    The Skhul and Qafzeh people were indeed tall relative to these other human samples, with male skeletal elements yielding stature estimates from 170-190 cm. The average stature of American men today is 176 cm. Holliday [2] showed that early Upper Paleolithic males had an average stature around 170 cm. According to Carretero and colleagues, the average Sima de los Huesos adult male had a stature around 168-170 cm. And as they note, taller individuals with stature estimates of 180 cm or more are present in the Early and Middle Pleistocene sample -- most notably the large Kabwe tibia, but we can also mention KNM-ER 1808 and KNM-ER 736 from the Early Pleistocene of Kenya.

    I disagree with the paper's suggestion that modern humans represent a new pattern of tall stature compared to earlier humans. I propose instead as a null hypothesis that human stature has not changed systematically since the Early Pleistocene.

    That doesn't mean human stature hasn't evolved. Human populations today are variable in stature, and they were in the recent past. We have pygmy populations with statures that average 150 cm or less in males, and peoples with statures that average close to 180 cm. Tall and short-statured populations today live in nearly every region, or did so in early Holocene times. Some of the variation in stature among populations is nutritional, some is additive, and both sources of variation appear to have emerged repeatedly in different contexts in recent human evolution.

    I suggest that pattern of variability would also have been present in earlier populations of humans. The differences between Neandertals and early Upper Paleolithic Europeans and the Skhul-Qafzeh sample were substantial but do not exceed the differences among recent human populations. The human stature adaptation is variable within a relatively broad niche, and has been so for nearly 2 million years.

    References

    1. Carretero J-M, Rodríguez L, García-González R, Arsuaga J-L, Gómez-Olivencia A, Lorenzo C, Bonmatí A, Gracia A, Martínez I, Quam R. Stature estimation from complete long bones in the Middle Pleistocene humans from the Sima de los Huesos, Sierra de Atapuerca (Spain). Journal of human evolution. 2011.
    2. Holliday TW. Body size and postcranial robusticity of European Upper Paleolithic hominins. Journal of human evolution. 2002;43(4):513-28.
    Tags: 
    Atapuerca
    Sima de los Huesos
    stature
    body size
    Neandertals
    Synopsis: 
    The long bones of the Atapuerca people double our information about early human statures

  • Steampunk genetics

    Thu, 2011-09-15 10:42 -- John Hawks

    An article in European Journal of Human Genetics that came out a couple of years ago has always impressed me, and I just noticed that it has gone to open access: "Predicting human height by Victorian and genomic methods" [1].

    The premise is that Galton's method of predicting stature from relatives still gives substantially better predictions than genotyping a collection of known variants that influence stature. It's basically a restatement of the "missing heritability" problem in more concrete (and colorful) terms. Here's a passage from the abstract:

    For highly heritable traits such as height, we conclude that in applications in which parental phenotypic information is available (eg, medicine), the Victorian Galton's method will long stay unsurpassed, in terms of both discriminative accuracy and costs. For less heritable traits, and in situations in which parental information is not available (eg, forensics), genomic methods may provide an alternative, given that the variants determining an essential proportion of the trait's variation can be identified.

    A great illustration. We know the trait is heritable, but the heritability is spread across many, many loci most of which remain unidentified. Hence, we can't predict the stature well from genotypes.

    References

    1. Aulchenko YS, Struchalin MV, Belonogova NM, Axenovich TI, Weedon MN, Hofman A, Uitterlinden AG, Kayser M, Oostra BA, van Duijn CM, et al. Predicting human height by Victorian and genomic methods. European Journal of Human Genetics. 2009;17(8):1070 - 1075.
    Tags: 
    stature
    missing heritability
    heritability
    GWAS

  • Statures of fossil Homo

    Tue, 2011-09-13 00:25 -- John Hawks
    Synopsis: 
    A laboratory exercise that applies regression equations to estimate the statures of some fossil hominin femora.

    Homo erectus and Neandertals were more or less human-sized. That may not be saying much, since we are so variable in stature ourselves.

    In this case, the fossils don't entirely speak for themselves. To estimate the sizes of ancient people, working with long bones, we must apply some kind of regression or other estimation method.

    1. KNM-ER 1481 is a complete femur from Koobi Fora, Kenya, approximately 1.9 million years old. Without any associated skull or teeth, we can't be sure what species it represents. Many scientists attribute it to early Homo because of its differences from known australopithecine femora.
    2. The Trinil femur was found by Eugene Dubois in 1892 as he excavated fossil beds at Trinil, Java. He had found a human skullcap the year before, and after finding the femur's humanlike anatomy, Dubois named a new species, Pithecanthropus erectus. This is the original Homo erectus femur. Today, we are less certain about its age and association with the partial skull. It may be a million years old, but it may be substantially younger.
    3. The femur from Spy, Belgium, represents a Neandertal who lived around 45,000 years ago. This femur is part of a more complete skeleton, and exhibits many of the characteristic features of Neandertal long bones, including the great thickness and curvature of the shaft and very large joint surfaces.

      4. What to do: Here you will examine the fossil cast femora, using regression equations to predict stature of the individual.

      1. Determine the sex of the individual. The femur head diameter is a relatively good indicator of sex. If it is less than 44 mm, the individual is likely to be a female. More than 46 mm, and the individual is likely to be a male. In between these values, you may need more information — either from the rest of the skeleton or from the size and robusticity of the femur itself.
      2. Measure the maximum length of the femur. This measurement is taken using the osteometric board, and represents the maximum distance from any points on the proximal and distal ends of the bone. Take your measurement in centimeters.
      3. Apply the correct regression equation. These are specific to sex and race. The femora at this station come from donated anatomy collections from the early 20th century, and represent people of European ancestry. The male and female regression equations for this population are listed at right.
    Study questions: 
    1. What are some weaknesses of estimating body size for fossil humans by applying a regression drawn from a contemporary human population?
    Study terms: 
    stature
    regression
    Koobi Fora
    Trinil
    Spy
    Neandertal
    Homo erectus
    Tags: 
    stature
    Anthropology 105
    explainer

  • Heritability and stature

    Mon, 2011-09-12 01:42 -- John Hawks
    Synopsis: 
    Heritability is the proportion of variance in a phenotype explained by additive genetic variance.

    Tall parents tend to have tall children.

    That's a simple generalization, not an absolute statement. You may be a short person with two tall parents. If you know many families, you'll probably know someone who is an exception to the rule. Two short parents may have a very tall daughter, and two siblings may be very different heights.

    Still, if we look at many families we will find that the stature of the parents lets us make some fair predictions about the statures of their children. We can look at data to quantify just how parent and offspring heights are related.

    For example, here is a plot of the statures of students in my Anthropology 105 class in 2010, compared to the mean of their mothers' and fathers' statures. The average of mother and father's heights are the midparent stature.

    Young men in this class have a taller average stature than young women. The picture separates men and women, and both taller sons and daughters tend to come from taller parents.

    We can do a bit better than this to quantify the relationship of the parents' and sons' and daughters' statures: We can put lines on the graph to show how the sons' and daughters' heights tend to increase with the midparent stature:

    Each of these lines is called a linear regression between midparent stature and offspring stature. The linear regression is the line that has the smallest squared distance from all of the points, added together.

    The squared distance is special. In statistics, the average squared distance from all points to the mean is called the variance. When we consider a trait like stature, there are many possible biological causes that can contribute to individuals being taller or shorter than the average. In statistical terms, these causes are all factors that contribute to the variance of stature.

    In the chart above, the linear regression represents the amount of the variance of the stature of the offspring that was contributed by the variance of their midparent statures. Parent statures can predict offspring statures and the linear regression is the prediction.

    It's not a perfect prediction. Take a look at the midparent value of 160 cm. When the midparent average is 160 cm, the regressions predict that daughters will be 156 cm and sons will be 168 cm. Out of Anthropology 105 students last year, two men had parents with an average of 160 cm, and both of them were very close to 168 cm in height — a good prediction! But the two women with parents this stature have very different heights. One is only 148 cm, the other 162, both more than 2 inches from the predicted value, in different directions. The regression gives the best prediction we can make from the parents, but there is obviously a lot of variation that can't be predicted in that way.

    Let's look more closely at the female students. The following chart adds together females from several years of Anthropology 105, with their midparent statures.

    The slope of the regression across these 300 women is 0.72. That means that roughly 72 percent of the variance of the students' stature can be attributed to variance in their midparent stature.

    This regression is special in genetics. Daughters resemble their parents because they inherit genes from them. The regression between the midparent and daughters' statures gives us a way to estimate the effects of those genes. In fact, the proportion of variance in a trait that can be explained by genes is the same as the slope of the regression. Geneticists call this proportion the heritability of stature. In my Anthropology 105 classes over the last few years, we would estimate the heritability of stature as 0.72, or 72%.

    Again, there are exceptions. Sometimes parents give other things to their children besides genes. The right foods, the right resources can make a difference to growth and development. And some genes can have unexpected effects. Most obviously, some genes make sons a lot taller than daughters, which is why we've considered the two sexes separately.

    The heritability of a trait is a powerful concept. It's important to understand some of its strengths and limits:

    1. Heritability refers to a population. A female in my Anthropology 105 class may be taller or shorter than her parents. We can't predict 72% of that student's stature; instead, 72% of the variance in the students can be explained by the variance among their parents.
    2. Traits with higher heritability have a lower influence from the environment. Traits that are highly influenced by the environment have a lower heritability.
    3. The response of a population to selection on a trait is determined by the heritability.
    4. Geneticists can look at other relatives besides parents to estimate heritability. Some of the strongest estimates come from comparing identical twins (who have the same genes) to fraternal twins (who share on average half their genes).
    5. Estimating heritability doesn't require us to know anything about the actual genes themselves.

    The last point is very important. We can quantify the influence of genes without knowing anything about which genes even affect a trait. Francis Galton invented the parent-offspring method of estimating heritability, more than twenty years before the word "gene" was coined. Remembering this helps to remind us that the concept of heritability is limited. It is not a guide to how genes work, it is a simple scale of which traits have a stronger or weaker genetic influence.

    Because it is a proportion, heritability varies only between zero and one. A trait with zero heritability is one for which none of the variance can be explained by the parents' variance.

    Heritability may be very low because the individuals in the population have little genetic variability. For example, an orchard of apple trees may consist of genetically identical individuals that are clones of a single parent tree. The apples (hopefully) all taste the same. But the trees may be very different in height, branch form, and the number of apples. These traits may be strongly influenced both by the environments of individual trees and by chance. By contrast, wild populations of oak trees are not made up of clones. The heights of individual trees are still strongly influenced by environments, but they may also be influenced by differences in genes.

    Study questions: 
    1. Make a list of three traits that have low heritability in humans. Why are these mostly influenced by the environment and not genes?
    2. Are there environments inhabited by human populations that would make the influence of genes seem to be less on some traits?
    3. Suppose that the linear regression between midparent and offspring for a trait has a slope of 0.56. What would you estimate as the heritability of this trait?
    Study terms: 
    stature
    heritability
    variance
    Tags: 
    heritability
    explainer
    Anthropology 105
    stature

  • Predicting stature from bone measurements

    Tue, 2011-09-06 01:00 -- John Hawks
    Synopsis: 
    A laboratory exercise involving measurement of femora and estimation of stature using regression formulae.

    Anthropologists have collected data from many populations in the world, showing the relationship between the parts of the skeleton and body size and stature. The long bones are the most important elements for estimating overall stature, because each of them contributes to a fairly large segment of the body's length.

    We can use regression equations to give an estimate of the stature from a single long bone. These estimates are not perfect — sometimes a person is taller than you might guess from his femur, sometimes shorter. Moreover, the relationship of bone length and stature varies among human populations, because of differences in body proportions. But allowing for error, the estimates of stature from long bone lengths are among the most important pieces of information we can gather in the process of identification.

    What to do: Here you will examine isolated femora, using regression equations to predict stature of the individual.

    1. Determine the sex of the individual. The femur head diameter is a relatively good indicator of sex. If it is less than 44 mm, the individual is likely to be a female. More than 46 mm, and the individual is likely to be a male. In between these values, you may need more information — either from the rest of the skeleton or from the size and robusticity of the femur itself.
    2. Measure the maximum length of the femur. This measurement is taken using the osteometric board, and represents the maximum distance from any points on the proximal and distal ends of the bone. Take your measurement in centimeters.
    3. Apply the correct regression equation. These are specific to sex and race. The femora at this station come from donated anatomy collections from the early 20th century, and represent people of European ancestry. The male and female regression equations for this population are listed at right.
    Study questions: 
    1. Applying regression equations to estimate stature is a primary component of forensic investigation of the skeleton. If you found a femur, what would you do?
    2. Why do the equations vary between males and females?
    Study terms: 
    regression
    stature
    femur
    Tags: 
    stature
    Anthropology 105
    explainer
    laboratory

  • Polygenic traits and directional selection

    Sat, 2010-09-18 13:41 -- John Hawks

    This has been an eventful week for those of us who study the dynamics of recent selection in humans. The most significant event was the publication of a paper describing genetic analysis of a long selection experiment in Drosophila. Although the experiment differs from most natural instances of selection in some important ways, the results give some very helpful corroboration that the recent human pattern of adaptive evolution was rapid and of an expected pattern for massive selection on many traits.

    Meanwhile, Jonathan Pritchard and Anna Di Rienzo have a short review in the current Nature Reviews Genetics [1], discussing the idea that a large fraction of adaptive evolution may be difficult to find with current genetic evidence.

    Their idea is that polygenic adaptations are unlikely to occur by successive "sweeps" of new adaptive mutations.

    It seems likely to us that, as in traditional quantitative genetic models, many — possibly even most — adaptive events in natural populations occur by polygenic adaptation. Polygenic adaptation could allow rapid adaptive shifts, yet would often go undetected using conventional methods for detecting selection. Moreover, polygenic adaptation is qualitatively different from the models of adaptive substitutions that dominate the population genetics literature.

    This is not a new idea, but Pritchard and Di Rienzo review it in a productive way, and the topic is worth some deeper thought...

    An adaptive genetic substitution is often modeled as an episode of logistic growth. A new mutation, initially in a single copy, increases exponentially in numbers until it is very common in the population. After this point, it continues to increase in frequency up to fixation, but progressively slowly. The entire process takes hundreds or a few thousands of generations, which sounds like a long time but is actually very rapid compared to the deep genealogical histories of most genetic loci. The initial rapid increase in numbers carries a region of linked sequence along with the selected variant. This "hitchhiking" region is highly visible because of the co-association of nearby allelic variants. Thus, if such a "sweep" is ongoing, we should have little trouble finding it. In humans we've found a lot of them, which is a big piece of evidence for the rapidity of human evolution during the past 40,000 years.

    But all that describes the dynamics of a single, strongly selected, mutation. What if a trait comes under selection, but the variation in the trait is explained not by a single gene, but by dozens or hundreds of genes? Pritchard and Di Rienzo outline such a scenario:

    The key point is that we should expect such an adaptation to occur by small allele frequency shifts spread across many loci. As a hypothetical example, consider the adaptation of human height — a trait for which there are probably hundreds of SNPs that each affect height by a few millimeters. Strong selection for increased height could be very effective, as height is extremely heritable. But at the level of individual SNPs, the effect of selection would be rather weak, exerting just a small upward pressure in favour of each of hundreds of 'tall' alleles. Suppose that at 500 SNPs, the tall alleles each increase the expected height of a person by 2 mm. Then, an average shift of just 10% in the population allele frequency of each tall allele would increase average height in the population by 20 cm (assuming that SNPs contribute additively). Although these numbers are hypothetical, they illustrate that, for a highly polygenic trait, a dramatic adaptive response could result from modest allele frequency changes at many loci. This model is different from classical sweep models. Most importantly, adaptation could occur without dramatic allele frequency changes and without adaptive fixation events.

    But the description isn't precisely what would happen in the case of selection on stature. Consider:

    1. It is true that alleles that already exist in the population provide the most immediate opportunity for change under directional selection. Any short-term phenotypic evolution we see is likely to be caused by changes in the frequency of standing variants.

    2. Some of the alleles that affect stature are constrained by their effects on other phenotypes. They might not change, even under directional selection on stature.

    3. Stature may be affected by hundreds of loci, but these do not account for equal proportions of the additive variance. Loci are subject to selection roughly in proportion to the additive variance in fitness they explain. Directional selection on stature will change the allele frequencies for a few loci quite a bit more quickly than most.

    The distribution of effect sizes is fairly well known for stature in humans. For example, Park and colleagues [2] this spring plotted the distribution of effect sizes for variants discovered by GWAS in 63,000 Europeans:

    Effect size distribution of variants found to explain heritability of stature, Crohns and BPC cancers in human genome-wide association studies

    In the figure, (a) is based on observed loci -- for stature, this includes 30 loci that reached significance in the GWAS without follow-up genotyping. There is a pretty severe ascertainment bias against small effect sizes, so curve (b) attempts to model the actual distribution correcting for ascertainment. Curve (c) is normalized to give the three conditions the same observed range.

    You can see that if we suddenly started selecting for height, most of the genetic response would come from a very small proportion of the loci that explain the current additive variance. These would be the subset of loci in the large-effect-size tail of the distribution, excluding those that are constrained by their role in other phenotypes under selection.

    4. As an allele becomes common enough (going up toward fixation), the locus will account for less and less of the additive variance in fitness. To maintain the same response to selection, other alleles must pick up the slack. Over time, groups of different alleles will come into focus of selection, sort of like the "cover flow" feature of an iPod. Some alleles increase in frequency across a transient in the mid-frequency range, only to be gradually replaced by others. Most of the phenotypic change occurs as alleles cross rapidly from 40 to 60 percent or so.

    5. A few loci will be special. These account for an appreciable fraction of additive variance even though the favored allele is very rare. As they become common, these favored alleles change in frequency more and more rapidly, and account for more and more of the additive variance. They suck up the oxygen of selection. These alleles will look like a classic sweep.

    6. Over many generations, new mutations may occur that also have strong effects on the trait. They will follow the "special" pattern described in 5.

    The question is how many loci of this type can we expect to exist? We all know that there are two patterns that could account for the heritability of traits like stature, where no common variants have very strong effects. Either the additive variance is spread across many rare variants with large effects, or instead across many common variants with small effects. Pritchard and Di Rienzo's scenario accentuates the second of these -- a small frequency change in many common variants with small effects.

    But if even a small fraction of the additive variance is explained by a few rare variants with strong effects, these may cause most of the phenotypic change, and may look a lot like a standard selective sweep.

    Pritchard and Di Rienzo note that the two options -- a rapid sweep of one or a few locus, versus slight frequency changes in many loci -- are not mutually exclusive. Most cases of directional selection on phenotypes may involve both patterns. If so, that will be very helpful, because we can use the easy-to-find sweeps to target analysis of harder-to-find frequency changes.

    They sketch a strategy for examining the evolution of such traits.

    One type of approach will be to identify phenotypes that may have undergone adaptive changes in particular environments, such as adaptations to cold climate, high altitude or novel ecological conditions. To dissect the genetic basis of such adaptations, one might collect phenotyped samples from closely related populations that have and have not experienced the selective pressure of interest and use GWA mapping to identify relevant quantitative trait loci (QTLs). Additionally, one would want to measure the extent of phenotypic adaptation — estimated as the difference in average phenotype between the adapted and non-adapted populations when they are living under matched conditions (exact matching of conditions may be difficult in human studies). Then one could ask: what fraction of the phenotypic difference can be explained by alleles with large versus small frequency differences? Are the phenotypic effect sizes of QTLs with large allele frequency differences greater than those with subtle frequency shifts10? What fraction of the phenotypic difference cannot be explained by detected sweep signals or QTLs at all (and hence might result from the cumulative effect of many weak QTLs)?

    In another type of scenario, one might hypothesize that a particular aspect of the environment is an important selective factor (for example, climate or diet) but it is unclear what all the relevant phenotypes are. In this case, we might study adaptation by looking at sets of populations that have independently adapted to the same selective pressures. One type of signal would be alleles that show parallel frequency shifts in response to similar environmental pressures in distantly related populations (although this type of approach is unlikely to be powerful for alleles with very small effects).

    These are exactly the kind of tests that we are working on here at Wisconsin. We have some pretty promising ideas, I think. If you're on a dissertation grant panel, would you please give some money to my students who want to apply these approaches?

    I mean, really, this is the best application of anthropology to develop new genetic approaches, rich in theory and in empirical evidence. Humans are the ideal model organism, because we know the histories and ecologies of different populations. Since the development of agriculture, we've had several ongoing natural selection experiments in our species.

    Nor can we ignore the longer prehistory of human populations. I tend to think that a lot of recent selection has involved new genetic solutions in cases of strong stabilizing selection. A trait like brain size does not evolve under classic directional selection, but instead as a consequence of shifting patterns of stabilizing selection. With intense selection on multiple functions, such traits are constrained in their evolutionary response. Slight frequency changes are not likely to relax such constraints, but a new mutation of large effect might break a long-standing genetic logjam.

    So I think Pritchard and Di Rienzo have outlined many important issues in this review. They have the potential to be highly productive for people with a little talent for applying theory to the data.

    References

    1. Citekey Pritchard:DiRienzo:2010 not found
    2. Park J-H, Wacholder S, Gail MH, Peters U, Jacobs KB, Chanock SJ, Chatterjee N. Estimation of effect size distribution from genome-wide association studies and implications for future discoveries. Nature Genetics [Internet]. 2010;42:570–575. Available from: http://dx.doi.org/10.1038/ng.610
    Tags: 
    fitness effects
    gene networks
    GWAS
    stature
    adaptation
    fitness landscapes
    gene-phenotype associations
    recent selection

  • The shrinking youth

    Fri, 2010-09-17 13:59 -- John Hawks

    Yesterday the Journal of Human Evolution released a new paper by Rhonda Graves and colleagues, titled “Just how strapping was KNM–WT 15000?” [1]. The paper challenges almost 25-year-old estimates for the body size of this important 1.5 million year old skeleton.

    For all this time, the textbooks have reported that early Homo in Africa had the same tall and elongated physique as current East African people like the Maasai. The new paper says that the textbooks are wrong -- the skeleton doesn't represent an individual who would have grown to be 6'1" (185 cm), instead it was near the end of its growth trajectory, for an adult height of around 5'4" (163 cm).

    That's a pretty massive change, and when the authors presented this work at the AAPA meetings last spring, it wasn't without controversy. So naturally we should look closely at the paper, understand its conclusions, and assess what this new estimate means for our understanding of early Homo. As you might guess from reading some of my earlier posts, I've been thinking that the body sizes of the rest of the Pleistocene record add up to a fairly simple picture. One of the few outliers from this picture was KNM-WT 15000. I'm inclined to think that the new estimate fits the bigger picture -- for example, I wrote this spring about "Shrinking erectus".

    Which means, of course, that I should be even more skeptical.

    KNM-WT 15000 was a juvenile at the time of death, and so any estimate of body size involves some assessment of the skeleton's state of development. This has presented a problem for assessing how much the individual had still to grow at the time of its death. The eruption and development of the teeth appear to be consistent with a fairly young age at death, by most estimates younger than 11 years, and by some as young as eight. That's using a human frame of reference. If we turn to a frame of reference drawn from chimpanzees or other apes, the estimated age at death from tooth development is even a bit younger. In contrast, the state of bone development seems to indicate a somewhat higher age at death: older than 11, and by some estimates as old as 15 years.

    KNM-WT 15000 skeleton

    Graves and colleagues, looking at this apparent mismatch between dental and skeletal development in this specimen, suggests that we need to look at a broader range of possible developmental models for early Homo erectus. A modern human developmental model is not a good fit, and neither is an ape developmental model. So their study involves creating a range of possible developmental trajectories for early Homo. These trajectories are based on data from living apes and humans, but altered by accelerating some phases or changing the intensity of the adolescent growth spurt.

    The growth spurt is very important to this issue, because it's one way that humans and most other primates differ greatly. Growth during that phase of development contributes disproportionately to the tall stature of modern humans. If Homo erectus didn't have the same kind of growth spurt as we do, then the stature of this specimen would have been a lot shorter than we would estimate for a human of the same age.

    The section of Graves and colleagues' discussion that covers the adolescent growth spurt is, to my mind, the central issue in the paper. Their review begins with a survey of literature on why a growth spurt exists. Most assume that there is some kind of trade-off between early weaning in humans, brain growth, and a large adult body size–with the optimal solution being slow juvenile somatic growth, fast juvenile brain growth, and they “catch up” of somatic growth during adolescence. Graves and colleagues assert that this pattern was not present in early Homo erectus, and that a more chimpanzee like growth spurt may be a better model.

    The velocity growth curves for human stature and chimpanzee total body length (summed length of crown-rump, femur, and tibia) highlight the difference between modern human and chimpanzee growth and development (Fig. 1). Both species exhibit growth spurts, but these spurts differ in rate, timing, and duration (Leigh, 1996). Pre-pubertal growth spurts in mass have been documented in many primates ([Tanner, 1962], [Laird, 1967], [Timiras and Valcana, 1972], [Leigh, 1996], [Leigh and Shea, 1996] and [Hamada et al., 1996]), but to date only slight increases in crown-rump length and total body length have been observed in chimpanzees (Hamada and Udono, 2002). Male chimpanzees (and possibly macaques) undergo a small growth spurt in length during the period between emergence of the first and third molars ([Watts and Gavan, 1982] and [Tanner et al., 1990]), but peak velocity is not as high and the growth spurt not as extended as in modern human adolescence. The velocity of chimpanzee growth decreases slightly between the ages of four and eight, and then begins to decline rapidly until adult total body length is reached at between 12 and 13 years of age. Chimpanzee growth spurts therefore differ in their onset, offset, and intensity compared to the modern human adolescent growth spurt (see Fig. 1; [Bogin, 1993] and [Bogin, 1996]). The growth spurts in the “ALH 12.3/25%” and “ALH 12.3/50%” curves approximate the juvenile pre-pubertal growth spurt exhibited by chimpanzees, which is of shorter duration and lesser magnitude than the full-blown modern human adolescent growth spurt. We contend that these curves most closely match what is currently known about growth and development in H. erectus but acknowledge that the data currently available limit our ability to choose a single curve. It is also possible that future studies documenting growth in wild chimpanzee length may provide evidence to support a different set of growth curves.

    Their small stature estimate for KNM-WT 15000 doesn't entirely hang on this point, but this assumption about the growth spurt makes more difference than any other single factor.

    We can reasonably ask: is there any other support for this assumption?

    The apparent mismatch between dental and skeletal developmental patterns in the specimen is consistent with the lack of a humanlike growth spurt. But evidence from the skeleton itself is weakened by the fact that KNM-WT 15000 appears to have suffered from some kind of growth pathology, as argued by Latimer and Ohman [2]. The pathology argument has mostly come into play over the issue of vertebral canal size in the specimen, but anything that affected skeletal growth may well have affected the relation between epiphyseal closure and dental eruption. Naturally, if the developmental pathology was a significant influence on growth, then we shouldn't be using WT 15000 as a model for early Homo erectus stature anyway.

    A more relevant argument is that KNM-WT 15000 is really an outlier when we assume that it would have grown to a very tall stature. On first appearance, this seems correct. We have quite a number of femora from Homo erectus, both inside and outside of Africa. Only two of them approach the length that had been estimated for the Nariokotome adult stature estimate. KNM-WT 15000's former adult estimate is the extreme.

    But looking more closely, both those tall individuals come from generally the same time and place as KNM-WT 15000. KNM-ER 1808 and KNM-ER 736 both preserve partial femur shafts with estimated lengths above 480 mm. Both specimens are a bit older than Nariokotome, between 1.6 and 1.7 million years old. KNM-ER 1808 in particular contributed heavily to the argument that early Homo erectus had a very tall stature, because the partial skeleton includes a fragment of pelvis, argued to be female. A tall woman makes for a very tall species.

    Still, these two specimens don't seem as significant in 2010 as they did twenty years ago. The Gona pelvis suggests that we don't really know the sex of KNM–ER 1808. Its pelvic fragment looks female in the context of living human dimorphism, but quite possibly male compared to the Gona individual. Henry McHenry [3] estimated adult statures for the KNM–ER 1808 and KNM–PR 736 femurs, both around 5'10" (180 cm). Those are the tall end of stature estimates for Homo erectus, both taller than average for living humans. But perhaps neither is surprising when taken as the largest and of the distribution that on the whole is relatively small bodied. An estimate of 163 cm for the adult height of KNM-WT 15000, as suggested by Graves and colleagues, would not be an outlier in this population, but neither would an estimate as large as 180 cm.

    So I think the comparative evidence is equivocal. Revisiting the specimen with a smaller estimate is reasonable, but I think our ability to assess the accuracy of any estimate is very limited. In light of the pathology of KNM-WT 15000, it may not be very relevant to understanding body size evolution in early Homo, anyway.

    The main problem facing us with understanding body size in early Homo is deciding which specimens should be included in which taxa. If we exclude everything except the relatively tall ones, like KNM–ER 1808 and KNM–ER 736, then we are going to end up with a tall stature estimate for a population, putatively H. erectus. But if we include some of the smaller specimens, like KNM–ER 993, or KNM–ER 803 – both contemporaries of the Nariokotome skeleton – than the average for this more inclusive population will be a lot lower. In East Africa 1.5 million years ago we can't assign an isolated femur to a species, and we won't have a good answer for this issue until we have many more associated specimens.

    I tend to think that small stature is the null hypothesis now, given our knowledge of the small stature of the Dmanisi hominins, and the moderate body size of middle Pleistocene Homo everywhere else. There are a few specimens that represent individuals as tall as those indicated by KNM-ER 736 and KNM-ER 1808, but none taller, and many much shorter.

    It's a much deeper topic than one skeleton, but the problems assessing stature in that skeleton help to highlight the difficulty of the problem in a global sense.

    UPDATE (2010-09-18): A reader suggests that I give a link to a 2004 paper by Shelley Smith, which compared the dental and skeletal maturation of KNM-WT 15000 to a large growth series of modern Canadians [4]. She found cases in the sample with comparable mismatches of dental and epiphyseal age estimates, and argued that we can't exclude a humanlike growth spurt for early Homo. That's one reason why I think this issue can't be resolved -- the variation in humans is great enough to encompass the known fossil specimens.

    A similar lack of resolution applies to enamel growth increments in KNM-WT 15000 ("Dental growth in early Homo"). The specimen can't be distinguished from Australopithecus, but the range in modern humans is very extensive.

    At the moment, skeletal correlates of growth don't give us the resolution to answer these questions definitively about early Homo. If we had more specimens, we could at least reduce the component of error from sampling, which would help considerably. But we can't expect that anytime soon.

    References

    1. Graves RR, Lupo AC, McCarthy RC, Wescott DJ, Cunningham DL. Just how strapping was KNM-WT 15000?. Journal of Human Evolution. 2010;59(5):542 - 554.
    2. Latimer BM, Ohman JC. Axial Dysplasia in Homo erectus. Journal of Human Evolution. 2001;40:A12.
    3. McHenry HM. Femoral lengths and stature in Plio-Pleistocene hominids. American Journal of Physical Anthropology. 1991;85:149–158.
    4. Smith SL. Skeletal age, dental age, and the maturation of KNM-WT 15000. Am. J. Phys. Anthropol. [Internet]. 2004;125:105–120. Available from: http://dx.doi.org/10.1002/ajpa.10376
    Tags: 
    body size
    Homo erectus
    Nariokotome
    Africa
    Early Pleistocene
    Kenya
    stature
    Synopsis: 
    The Nariokotome skeleton once defined the tall linear body form for early Homo. Now it's 5'4".

  • Shrinking erectus

    Tue, 2010-04-27 10:02 -- John Hawks

    Ann Gibbons reports on the AAPA meetings with a story about all the Homo erectus pelvis and stature papers ("Human ancestor caught in the midst of a makeover," subscription required). Research on the proportions of early Homo was the main event of the meetings, and Gibbons really caught the highlights of the story.

    I wrote about body size in Homo erectus a few months ago, and much of the story follows from the basics I outlined there ("The changing height of Homo erectus"). But there I emphasized that the estimated adult height of KNM-WT 15000 was an outlier in a relatively small body size distribution.

    What I didn't anticipate is that some interesting work might come along to question the tall adult stature estimate for that skeleton. Gibbons describes the work of Ronda Graves and colleagues, presented at the meetings:

    Using intermediate growth rates, graduate student Ronda Graves of Stony Brook University in New York state calculated that Nariokotome Boy would have had less time than originally predicted to reach his adult height when he died. She estimated at the meeting that he would have reached 163 cm in height and 56 kg in weight as an adult—"shorter and wider" than previously thought.

    This seems very short, at least when I first saw it. On reflection, Ohman and colleagues (2002) had provided a stature estimate at death of KNM-WT 15000, as only 147 cm, and they suggested it might have been as short as 141 cm. That's an awful lot shorter than had previously been estimated on the basis of regressions.

    If Graves and colleagues are right about the lack of a human-like growth spurt, an additional 20 cm (8 inches) wouldn't be unusually small for an adult stature. Those stature estimates would put KNM-WT 15000 between the 50th and 90th percentiles for American 10-year-old boys, or between the 25th and 75th percentiles for 11-year-olds. By contrast, an adult stature of 163 would be around the 3rd percentile for adult American men. The assumptions about growth totally determine the outcome for adult height.

    The credibility of the growth assumptions can only be tested by looking at other adult and juvenile remains. There is much more to say on this topic, but I'll point out one relevant comparison: The estimated stature of the adult skeleton from Dmanisi, including the complete D4167 femur and D3901 tibia, is between 145 and 166 cm. Graves' KNM-WT 15000 stature estimate is right within this range.

    Meanwhile, there was a lot of disagreement about hips.

    [Scott] Simpson and Linda Spurlock of the Cleveland Museum of Natural History realigned the pieces of Nariokotome Boy's pelvis, guided by a female H. erectus pelvis from Gona, Ethiopia, that Simpson reported 2 years ago (Science, 14 November 2008, p. 1089). They found that the widest measure from side to side on the boy's pelvis is 255 to 260 millimeters rather than 225 to 230 mm. This would give the boy an adult hip breadth of 295 to 301 mm rather than the 266 mm originally proposed, and would match those of the short, wide-hipped female from Gona, whose pelvic breadth was 288 mm. "H. erectus was not simply a small-brained modern human," says Simpson.

    Simpson's reconstruction seemed reasonable, and it's actually not that big a difference -- roughly an inch and a half (3 cm) in bi-iliac breadth. The main differences were in the overall shape of the pelvis, being shorter with a more flaring iliac blade.

    Gibbons describes the disputation that happened after Chris Ruff's presentation. Ruff has suggested that the Gona pelvis may not represent Homo -- that its broad proportions and small acetabula (hip sockets) suggest it may have belonged to an australopithecine (presumably, A. boisei).

    Much of the disagreement comes down to the estimation of femur head diameter from acetabulum breadth -- Ruff (2010) gave an estimate of 32.6 mm, Simpson and colleagues estimated between 35 and 36 mm, based on a different method. What you would want is enough acetabula of both genera to be able to examine their variation directly. We don't have such a sample; what we have are a few acetabula and several femur heads. We have the additional problem that living people seem to have a different relation of femur head and acetabulum diameters than in other anthropoids, and it's not obvious which should be applied to early hominins.

    I guess (in the relative absence of data) that this acetabulum diameter of the Gona pelvis was in the zone of overlap between Homo and Australopithecus. There's no question that later Homo -- say after 1 million years ago -- is substantially larger in acetabulum diameter, from every specimen so far described. But there are occasional small specimens of Homo even in the Middle Pleistocene. At 1.15 million years old, the Gona specimen is more than 300,000 years later than the last known occurrence of Australopithecus. The femur head that would fit the Gona acetabulum would be smaller than KNM-ER 1472 or D4167 from Dmanisi, both around 40 mm. At least one australopithecine femur head (AL 333-3) is that large, so the femur head diameter distributions do overlap. The STW 431 acetabulum diameter is a sliver larger than that of the Gona pelvis (Ruff 2010 makes it 3 mm bigger, but other workers have given a smaller estimate). SK 3155 may well be Homo and has a smaller acetabulum.

    Of course, if we go as far as SK 3155, we have to consider the topic of the Malapa innominate. Can we tell small-bodied Homo from Australopithecus on the basis of pelvic morphology? Several people writing about the Gona pelvis have made it sound like a bigger version of Lucy's. But that's not really true. The australopithecine-like appearance comes from its breadth and consequent features, including the long pubes and flaring anterior ilia. The rest? Maybe there's something here for a clever anatomist.

    UPDATE (2010-04-27): I have some e-mail about the last occurrence of A. boisei, which I wrote above was more than 300,000 years older than the Gona pelvis.

    The most potent counterargument is Swartkrans Member 1, which has uranium-lead dates around 830,000 years ago, and has been placed by many workers around a million years ago. I actually hadn't been thinking of South Africa. But it is relevant, as the East African record between 1.4 and a million years ago may not be strong enough to argue that the last occurrence of A. boisei is really very close to the extinction time.

    Meanwhile, there is OH 36, an ulna from Olduvai Gorge that may represent A. boisei. Since it's (obviously) not cranial, and is quite large and robust compared to postcranial remains that are associated with A. boisei, I've always been very skeptical of that assessment. If there's one feature of the ulna that actually has some phylogenetic importance in the Early Pleistocene, I figure it's size.

    But given the current question about body size, that reason for skepticism may have receded in importance. On the other hand, OH 36 seems to represent a substantially bigger individual than the Gona pelvis, so maybe introducing robust australopithecines into the mix doesn't help anything.

    Several things puzzle me. Even into Member 1 times, Swartkrans is dominated by A. robustus, with very little Homo. In East Africa, A. boisei is never quite so predominant in the hominin assemblage as the case in South Africa, but was nevertheless very common up to 1.5 million years ago. Did it persist much later? Was it cryptic from the point of view of the fossil record? Are the Swartkrans dates older than we think?

    References:

    Gibbons A. 2010. Human ancestor caught in the midst of a makeover. Science 328:413. doi:10.1126/science.328.5977.413

    Ohman JC, Wood C, Wood B, Crompton RH, Günther MM, Yu L, Savage R, Wang W. 2002. Stature-at-death of KNM-WT 15000. Hum Evol 17:129-141. doi:10.1007/BF02436366

    Ruff C. 2010. Body size and body shape in early hominins -- implications of the Gona pelvis. J Hum Evol (in press) doi:10.1016/j.jhevol.20 09.10.0 03

    Tags: 
    body size
    stature
    East Africa
    Homo erectus
    Malapa
    A. robustus
    pelvis
    Early Pleistocene
    Swartkrans
    Gona
    KNM-WT 15000
    A. boisei
    meetings
    Synopsis: 
    The 2010 AAPA meetings featured a fight about the Nariokotome and Gona pelves.

  • Mailbag: Variation in stature

    Thu, 2010-01-21 15:32 -- John Hawks

    Re: "The changing height of Homo erectus":

    I was looking through today’s Redeye (Chicago Tribune’s mini paper) and saw a picture of the world’s shortest man (He Pingping of China) holding the finger of the worlds tallest man (Sultan Kosen of Turkey) and I thought if a paleoanthropologist dug these two up near each other they would never assign them to the same species. I wonder just how many finds have been misinterpreted and are actually the same species. I have definitely come around to the idea of anagenesis and I wonder if you could squeeze in something about Homo heidelbergensis perhaps in one of your post on Neanderthals or something. Did they evolve into moderns or die out?

    Well, H. heidelbergensis is certainly a can of worms.

    Here's a start: If we and Neandertals descend from a single population that lived 350,000 years ago or so, where did that population live? If Africa, what does that make the Sima de los Huesos sample, at 600,000 years old? If Europe + Africa, then why was its genetic variation so low, and how was this continuity maintained? And, if it was, why would we assume modern humans aren't part of this continuous population?

    This is the difficulty.

    Meanwhile re: variability -- we may be near that point already. Consider that "H. erectus" now includes adult specimens with endocranial volume of 600 ml and others with 1200. That doesn't exceed the extremes of normal human variation, but it is very unlikely you'd find this amount of difference in an equivalent sample size of humans. Or chimpanzees. Or gorillas.

    Tags: 
    Homo heidelbergensis
    stature
    Homo erectus
    Neandertals

  • The changing height of Homo erectus

    Wed, 2010-01-13 14:00 -- John Hawks

    Gretchen picked up a partial set of Time-Life volumes, from 1973, part of the series "The Emergence of Man". She found them at a garage sale. There's a lot of fun stuff in them, and some very useful illustrations.

    For example, I'm looking through the volume titled, The First Men, which is basically about Homo erectus. The meat of the book is a series of descriptions of fossil and archaeological finds -- Dubois on Java, Terra Amata, Torralba and Ambrona. No surprise, each of these has a very different theme than we would give them today!

    Here's a fun comparison:

    Body proportions of fossil hominins, from 1973

    This is a two-page spread in the book; really a fine illustration by Roger Hane.

    Homo erectus, in the middle, is reconstructed with a stature pretty much right in between Australopithecus and Homo sapiens.

    If you open up most recent textbooks, you'll find Homo erectus illustrated as the same height or taller than us. This is mostly due to the KNM-WT 15000 (Nariokotome) skeleton, discovered in 1984. This skeleton was estimated to have a moderately tall adult stature -- around 185 cm (6 feet 1 inch). There are three or four other femora from the Lower Pleistocene that also correspond to stature estimates up around 180 cm, in particular KNM-ER 736 and KNM-ER 1808.

    Now one might reasonably wonder, what's the big deal about 185 cm? The Nariokotome skeleton hardly represents a giant -- at 6'1" he would have been an inch shorter than me, for goodness' sake! And with only one fossil specimen within 2 inches, it shouldn't be churlish to point out that the Nariokotome estimate is not based on a real femur length -- it's an estimate based on an estimate. Most Lower Pleistocene fossil femora were much shorter, and yield stature estimates well under 180 cm. So why did anthropologists so eagerly cling to the tallest estimates for Homo erectus?

    Few Lower Pleistocene postcranial bones are associated with skulls, so it's difficult or impossible to assign smaller bones to a species. How do we know whether a short femur belongs to Homo erectus or Homo habilis -- which we know from OH 62 is much smaller in body size? Or A. boisei, which it would appear from KNM-ER 1500 is also smaller? We don't really know -- so the bones that correspond to mid-range stature estimates, say around 160 cm, might belong to any of the above. But the tall ones -- well, we know that those must represent the largest-bodied hominin. So there was a tendency to assume that the tall specimens were near the average for Homo erectus.

    It was a hypothesis. It has turned out to be false.

    The illustration in the Time-Life book is based on entirely different fossils. The Turkana fossils were unearthed during and after the early 1970's. Before that, Homo erectus stature could be estimated from the Trinil (Java) and Zhoukoudian (China) femora. These are later than the early African Lower Pleistocene sample. The Zhoukoudian femora in particular give stature estimates at or under 5 feet (152 cm). We might read it as a decline, and some people did as recently as 5 years ago. But the Dmanisi postcrania are also short, a bit shorter than the Zhoukoudian femora. And they're earlier than Nariokotome. And we now know of smaller crania of Homo erectus in the East African Lower Pleistocene. So the tallest statures aren't the average; they're the tallest.

    It ain't rocket science, I know. But this is progress.

    Today, I think it's fair to say that the variation of stature in Homo erectus was more or less like the variation within living people. There are short and tall populations today, varied in ecology and latitude. The average stature of young men in the Netherlands today is 184 cm. Adult women in the Philippines average only 150 cm. So the best way to compare statures is to illustrate the range.

    That being said, I don't think we know how stature has evolved over time. We do have some data points -- the Neandertals were shorter than Upper Paleolithic Europeans, for example, but seem to have been around the same height as Mesolithic people (and a shade taller than Neolithic Europeans). The Dmanisi people were on the short end of the human range, but not unusually so. The variability within Lower Pleistocene East Africans seems high, but I'd want to see a serious test compared to human populations.

    It's a case in the fossil record where discovering more seems to have resulted in us knowing less. But that's just because we can now reject several categorial statements that people used to accept uncritically.

    Related articles:

    "News flash: Dmanisi hominids were not short"

    "Body size in Holocene South Africa"

    Body mass in ancient humans and high-latitude populations

    Tags: 
    Zhoukoudian
    Dmanisi
    Nariokotome
    Turkana
    Homo erectus
    stature
    Synopsis: 
    The re-evaluation of the stature of KNM-WT 15000 provokes a "blast from the past", looking at the statures of other Homo erectus specimens.
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Neandertals

For years, I've worked on their bones. Now I'm working on their genes. Read more about the science studying these ancient people.

Denisova

From a finger bone of an ancient human came the record of a completely unexpected population. My lab is working on the science of the Denisova genome.

Acceleration

The advent of agriculture caused natural selection to speed up greatly in humans. We're uncovering some of the ways that populations have rapidly changed during the last 10,000 years.

Malapa

Just outside Johannesburg, the Malapa site is producing some of the most exciting finds in human evolution. This site is the headquarters of the Malapa Soft Tissue Project.