Homo habilis

In Nature a couple of weeks ago, Robin Dennell and Wil Roebroeks had a provocative paper exploring the possibility that early humans (i.e. Homo erectus) originated in Asia rather than Africa.

The paper is all speculation of course; there is no evidence of any earlier hominid in Asia.

But it is the good kind of speculation. Although maybe not quite this big:

Most probably, we are on the threshold of a profound transformation of our understanding of early hominin evolution that might prove as far-reaching as the demise of the notion of Man the Hunter in the early 1960s (Dennell and Roebroeks 2005:1103).

Here's the abstract:

The past decade has seen the Pliocene and Pleistocene fossil hominin record enriched by the addition of at least ten new taxa, including the Early Pleistocene, small-brained hominins from Dmanisi, Georgia, and the diminutive Late Pleistocene Homo floresiensis from Flores, Indonesia. At the same time, Asia's earliest hominin presence has been extended up to 1.8 Myr ago, hundreds of thousands of years earlier than previously envisaged. Nevertheless, the preferred explanation for the first appearance of hominins outside Africa has remained virtually unchanged. We show here that it is time to develop alternatives to one of palaeoanthropology's most basic paradigms: 'Out of Africa 1' (Dennell and Roebroeks 2005:1099).

It is worth reviewing exactly what "Out of Africa 1" is supposed to be. The paradigm is that emergence of hominids from Africa required increases in brain size and/or body size, coincident with the emergence of hominids like KNM-ER 3733, KNM-WT 15000, and others. The motivation for this hypothesis is simple: australopithecines have not been found outside of Africa. Nor has anything like Homo habilis, which is australopithecine-sized but has larger brains.

Of course, it is questionable just how basic this paradigm is. Consider what I (and my colleagues) were able to write only seven years ago:

The problem is that significant range expansion out of Africa occurred a half million years or more later than the first H. sapiens [corresponding to others' H. erectus or H. ergaster]. Population size before then may have remained small, and this is not an inconsequential time span, being one quarter of the time H. sapiens has existed. An important date in behavioral evolution is 1.5 MYA because it is marked by the earliest appearance of the Acheulean, the ubiquitous hand-axe industry of the Early and Middle Pleistocene.... Before this time, humanity was limited to Africa and immediately adjacent sections of Asia such as the Levant (Hawks et al. 2000:7).

Evidence for large body size in Late Pliocene humans (notably KNM-WT 15000 but also many others) made it very plausible that larger bodies were necessary for dispersal from Africa. But without good evidence for such dispersal before around 1.4 million years ago (and arguably not before 1 million years), larger bodies could not be assumed to be a sufficient condition for dispersal. Writing about the origin of humans, we had to consider all these alternatives -- at a time when the Dmanisi sample consisted of a single uncertainly dated mandible and the Mojokerto date stood alone with very questionable provenience.

Now we know that hominids did leave Africa by at least 1.8 million years ago. Dmanisi has almost singlehandedly changed the perspective.

And in doing so, it made much more convenient the hypothesis that large body size was both necessary and sufficient for dispersal from Africa. If the date of dispersal and the date of human origins are the same, then it is natural to propose that the coincidence is more than chance.

I would say this is more of a convenient hypothesis (and an easy story to tell) than it is a basic paradigm. The idea that large body size caused dispersal from Africa may have been a local minimum in terms of parsimony (at least as long as the body size of the Dmanisi fossils was not known), but it was only one alternative among many still in play.

And it remains a plausible hypothesis -- after all, the Dmanisi remains are a bit larger than australopithecines, and they might well have shrunk from a larger early-human-like size after reaching Asia instead of before.

But Dennell and Roebroeks give motivations for examining some alternatives.

The only reason why the earliest tool assemblages in Asia are attributed to H. erectus s.l. is that palaeoanthropologists have already decided that, in effect, it was the only hominin capable of migration out of Africa, and with sufficient Wanderlust to do so (Dennella and Roebroeks 2005:1099).

Homo erectus sensu lato (s.l.) means Homo erectus "in the loose sense", which would include not only the "strict sense" (sensu stricto) H. erectus. from Java and China, but also hominids like OH 9 and KNM-ER 3733 from Africa, and presumably the Dmanisi hominids.

A long passage reviews the total faunal evidence from Asia during the Late Pliocene. The thrust of the passage is that there are very few sites with extensive fauna, and of these most preserve mainly large-bodied herbivores. There are a few hints that a hominid-friendly fauna may have existed, including the presence of baboons. But there are no hominids of any kind at the vast majority of Asian localities -- Dmanisi is a real exception in the Plio-Pleistocene record.

This is the key taphonomic argument: if we have only found Early Pleistocene humans from continental Asia within the past ten years, then how can we preclude there having been australopithecines there? Dennell and Roebroeks argue that if there were australopithecines, we shouldn't necessarily expect to have found them yet -- we just haven't looked extensively enough.

A close read of the section raises a caution, though. One of the main arguments for the incompleteness of the Asian record is that sites don't preserve each others' fauna.

It is also likely that the full range of taxa is incomplete for the Indian subcontinent, because Megantereon and Pachycrocuta are not recorded in India but are present in Pakistan; in Pakistan, there is no evidence of Camelus and small primates, and in neither country is Homotherium recorded, although this is present to the west at Dmanisi, to the north at Kuruksay, central Asia and to the east at Longuppo, south China (Dennell and Roebroeks 2005:1100).

Of course, all of these species are recorded in Asia taking all the sites in aggregate; this is hardly an argument for the overall weakness of the record -- just an argument that no individual site is an adequate record of the continent's fauna.

To me, the important question is not whether australopithecines as currently known from Africa were in Asia. A more troubling possibility is that the australopithecines that we now know from Africa were not the only (or main) manifestations of early hominids in Africa. Large parts of Africa that we might expect to be congenial to hominids, like the Zambesi basin, have few or no fossils at all. The recovery of the Bahr el Ghazal mandible (Brunet et al. 1994) certainly makes clear that hominids were living across a much larger area than we have adequately sampled. But that mandible is, although not identical, certainly very similar to known contemporary hominids in its adaptation.

The question is whether hominids had adapted to other ecologies that are much less satisfactorily sampled than the East African rift. They probably weren't living where chimpanzee and gorilla ancestors did, but where else might they have been? Some such ecologies -- like the coasts -- would make early dispersal very plausible.

(In this regard, early humans are not the only hominids who lack a satisfactory ancestor. Who was the ancestor of A. aethiopicus? In what ecology did the first robust hominid arise?)

So what is the broader set of hypotheses that we should consider? Dennell and Roebroeks suggest:

If the above taphonomic review suggests that we cannot show the absence of hominins from areas in Asia at a time before the little evidence we have indicates their presence, we need to consider alternatives to the current Out of Africa [that is, their "Out of Africa 1"] model. There are three issues here. The first is when hominin(s) first left Africa -- might they, for example, have left shortly after they acquired the ability to make stone tools, the earliest of which are currently 2.6 Myr old? Or could they have left even earlier, about 3.0Ð3.5 Myr ago, when some australopithecines were already living in the African grasslands? The second issue is whether we yet know the full range of hominins that inhabited both Africa and Asia in the Late Pliocene and Early Pleistocene. Even in east Africa, several new taxa have been claimed in the past decade (for example, A. anamensis, A. garhi, Ardipithecus ramidus and Kenyanthropus platyops) and doubtless more will be found. (An indication of how little we know about Pleistocene east Africa is that only recently has the first fossil evidence for chimpanzee been found.) In Asia, the recent discoveries of H. georgicus and H. floresiensis should make us very wary of assuming that H. erectus s.l. was the only player on the Asian stage in the Early Pleistocene. Third, Asia might not have been the passive recipient of whatever migrated out of Africa but might have been a major donor to speciation events, as well as dispersals back into Africa. Such two-way traffic is well documented for other mammals in the Pliocene and Early Pleistocene, such as Equus and bovids, with more taxa migrating into than out of Africa. There is no reason why hominin migrations were always from Africa into Asia, and movements in the opposite direction might also have occurred, as has been suggested for the Olduvai OH9 (refs 13, 58) and Daka specimens. We should even allow for the possibility that H. ergaster originated in Asia and perhaps explain its lack of an obvious east African ancestry as the result of immigration rather than a short (and undocumented) process of anagenetic (in situ) evolution (Dennell and Roebroeks 2005:1100-1101).

Of course, most of the evidence indicating the presence of hominids is not fossil but archaeological. On this topic, Dennell and Roebroeks have much to say:

Any stone tool assemblage in Asia dated as older than 1.9 Myr ago (the earliest date that Homo is supposed to have left Africa) is either dismissed or (more usually) ignored; undated Oldowan tools are assumed to date from after 1.9 Myr ago and not from 2.6 Myr ago (the date of their first appearance in east Africa); and stone tool assemblages in Asia dated to the Olduvai Event (1.77Ð1.95 Myr ago) and not associated with hominin remains are automatically attributed to Homo erectus s.l. However, there is no reason why Oldowan assemblages in Arabia cannot be older than 1.9 Myr old, or why the tools from Ain Hanech (Algeria) or Erq el Ahmar (Israel) were made by H. erectus s.l. [instead of other hominids] (ibid:1102, references omitted).

There is a section about what exactly absence of evidence can tell, a short critique of using continents as proxies for biogeographic units:

As noted earlier, Pliocene grasslands extended all the way from west Africa to north China, and 'Savannahstan' might prove a more useful spatial unit for modelling early hominin adaptations and dispersals within them than simply an undifferentiated 'Africa' or 'Asia'. For example, the African hominins 1.9Ð1.7 Myr ago at Koobi Fora (Kenya) and Ain Hanech (Algeria), and their slightly later counterparts in Asia at 'Ubeidiya (Israel), and Majuangou (north China) were all living in broadly comparable grassland environments, and it makes sense to place them within the same frame of reference.

I think there is much of value to consider here; but it is less a revolution and more a statement of the field in transition. There are also alternatives that are not considered in this paper but that may be equally plausible -- most notably, the idea that early humans themselves may have been substantially polymorphic (witness KNM-ER 42700), or that brain size rather than body size may have been a prerequisite to dispersal (since habilines, Dmanisi, and H. erectus s.l. are all allometrically similar in brain size).

National Geographic News also has an article about the paper.

References:

Dennell R, Roebroeks W. 2005. An Asian perspective on early human dispersal from Africa. Nature 438:1099-1104. Full text (subscription)

Hawks J, Hunley K, Lee S-H, Wolpoff M. 2000. Population bottlenecks and Pleistocene human evolution. Mol Biol Evol 17:2-22.

This week's Nature is carrying a paper by Morwood, Brown, and colleagues (2005) presenting additional skeletal material from Liang Bua as well as a commentary by Daniel Lieberman. Thanks to a reader, I found the permission slip from Nature lifting the embargo, so I can let fly without bogarting the kind journalist who forwarded me the paper.

What is noteworthy about the new bones?

The paper discusses three important specimens. The first is the adult mandible LB6/1. In its overall size and morphology it is similar to the mandible of LB1, reported last year. Like LB1 it lacks a chin and Morwood et al. (2005:1013) compare its symphyseal morphology to Dmanisi D211. Overall, the mandible is slightly smaller in tooth size and corpus size compared to LB1, and its ramus is quite a bit shorter.

LB6/1 is part of a partial skeleton. The other elements are not described in the paper, but they are listed: a portion of proximal ulna, a partial right scapula, a foot bone, one each of finger and toe bones, and a complete radius 157mm long. That's a short radius -- barely more than 6 inches. It was broken during life and healed.

Wait a minute. Did you say a 6-inch long radius?

Funny how nobody else seems to have picked up on this yet.

The authors estimate a brachial index (radius to humerus) for LB1 of 78 percent, estimating likely radius length from the ulna. This would put LB1 within the range of "tropical" human populations. If the LB6 individual had the same "tropical" brachial index, its humerus would be around 200mm long. That's 43mm shorter than LB1.

This admits a couple of explanations:

1. LB6 was simply a smaller individual than LB1. The mandible is more or less consistent with this hypothesis, which may therefore be the most likely.
2. LB6 did not have the unusually long arms of LB1. Where LB1 is australopithecine-like, perhaps LB6 was more humanlike. This seems less likely, but it would be consistent with the idea that the proportions of LB1 represent some kind of pathology.

Wait a minute. Did you say australopithecine-like proportions?

Yes, the LB1 humerus and ulna are relatively long compared to the femur:

For example, the humerofemoral index of 85.4 is outside the range of variation for H. sapiens, but is the same as AL 288-1 A. afarensis, and midway between the indices for apes and humans. The more complete left ilium [pelvic bone] also indicates that the pelvis is flared antero-laterally, consistent with an australopithecine-shaped thoracic region. Body proportions of LB1 are the same as AL 288-1 A. afarensis, but differ from all other hominins for which they are reliable data, including H. erectus (Morwood et al. 2005:1016).

"Outside the range of H. sapiens" is also outside the range of any Pleistocene human, by the way.

Didn't you say this was an australopithecine when it came out a year ago?

Well, yes. My first post on the subject was titled, "Liang Bua: an australopithecine from Flores?" And I did present a rationale for believing that the skeleton was australopithecine rather than Homo. My point initially was that the combination of small body size and relatively small brain size was very simple to imagine as a descendant of an australopithecine, but very difficult to imagine in a descendant of Pleistocene Homo.

Some other features of the skeleton resemble Australopithecus. None of them individually is sufficient to label the skeleton as australopithecine, but together they are suggestive. For example, the pelvis is broad, with a very prominent anterior superior iliac spine. It's very similar to australopithecines like AL 288-1 (Lucy) or Sts 14.

But we don't know to what extent the breadth and morphology of australopithecine pelves are consequences of their phylogeny as opposed to allometric consequences of their small body sizes. In other words, LB1 might look australopithecine-like because it is small, instead of actually being an australopithecine.

The postcanine teeth are relatively large for a human, but they are far from australopithecine-like in size. Aside from their size, the first molars are the largest; just the opposite of the australopithecine condition. The roots of the premolars are completely uninformative, since both australopithecines and early Homo have the bifurcated roots found in both Liang Bua mandibles. So if this was an australopithecine-derived population, it had evolved considerably smaller teeth. Happily, this evolution of smaller teeth might also account for the gracile, Homo-like facial morphology.

OK, so it's an australopithecine, right?

Maybe. It would not only have to be an australopithecine; it might have to be a DWARF AUSTRALOPITHECINE.

Consider that the femur length of LB1 is just a millimeter shorter than Lucy and its body proportions are basically the same. Lucy (AL 288-1) is not only the most complete known australopithecine skeleton (barring STW 573, which is yet to be described), it has the smallest limbs. There are some individual bone fragments with smaller dimensions than Lucy's, but not very much smaller. At the same time, there are many larger specimens. Some of these, like the Sibilot radius KNM-ER 20419, are a whole lot larger.

Now at Liang Bua, LB1 is nearly the biggest specimen. Brown et al. (2004) do report another radius from an older part of the deposit with an estimated length of 210mm. Again assuming the same brachial index, this would correspond to a humerus of 269mm, around an inch longer than LB1.

But the other two adult long bones reported are the LB6 radius (157mm) and the LB8 tibia. At an estimated 216mm, this tibia is substantially shorter than the 235mm LB1 tibia. There is no comparably complete australopithecine femur, but if Lucy (missing around a third of the shaft) was around the same length as LB1, then LB8 would be shorter than any australopithecine.

Even worse, it is shorter than all but one of 47 chimpanzee tibiae in my comparative data. That's really short.

So as it stands, it appears that the Liang Bua sample is substantially shorter than australopithecines. At the same time, remember that the brain size of LB1 (estimated by Falk et al. 2005 as 417 ml) is smaller than all but three australopithecines (KNM-WT 17000, AL 162-28, and AL 333-105). Together with the facial and tooth reduction, this is good evidence for selection for smaller size in an australopithecine-like population.

OK, so that rules out any chance that it is a dwarf modern human population, right?

Probably. This would have to be an exceptionally short sample of an exceptionally short population. But then it partly depends on how accurate the stature regressions are. As you get to the bottom of the size range of skeletons from which a regression is calculated, you get less accurate body size estimates. And the proportions affect the estimates. These deserve to be gone over carefully.

And there is no reason in principle why a modern human population could not have been smaller than any pygmy populations of today.

There are two stumbling blocks to the hypothesis that this sample represents a dwarf modern human population. The first is the fact that neither mandible looks modern. It is very hard to argue that the features of these mandibles are the consequence of small body size alone; they genuinely appear archaic.

The second is the size of the brain.

Speaking of the brain, is it small enough to rule out descent from early Homo?

That's a very good question. Large-bodied early Homo appears to have an average endocranial volume around 800 ml. A reduction in body size to LB1 should not have cut the endocranial volume in half -- we would expect a volume closer to 600 ml. In fact, an average of around 600 ml is just about what we observe for small-bodied early Homo, including H. habilis in Africa, and possibly including the Dmanisi sample of early Homo from the Republic of Georgia.

To get from a habiline-sized hominid to LB1 body size would take relatively little size reduction. This means that the brain size of LB1 would be very surprisingly small for a habiline of its body size (particularly since many habilines are its size, yet have much larger brains).

So to go from any variety of early Homo to LB1 in brain size would require pretty substantial selection for smaller brains. It is hard for me to see that happening in a hominid population, because it would likely lead to functional compromise of some kind. In particular, if most of the selection for larger brains in hominids has been to promote social intelligence, it is hard to see how selection for smaller brains would happen.

On the other hand, who knows? There is, after all, the endocast shape, which is Homo-like (Falk et al. 2005). And the facial morphology. And the teeth.

In the face of all this, Morwood et al. (2005) appear to be persuaded most strongly by the limb proportions. Maybe an australopithecine-like limb ratio is a good phylogenetic indicator, but considering the recent spat over early hominid limb proportions in Current Anthropology, this might not be the best hook to hang your hat on.

Didn't you say six months ago that LB1 is pathological? Well, what do you say now, smarty-pants?

I still think it's pathological. We have so far seen a few of the details that point to that conclusion. For example, there's the low torsion of the humerus. "Torsion" refers to the angle between the axis of the head and the axis of the distal end of the bone. The humerus of LB1 is unlike any hominid. It's unlike any great ape. It's like monkeys and gibbons. That's weird. Then there is the bowed tibia, and the rotated premolars, all clearly in the reports so far.

We will see if these and other features can be combined into a single diagnosis of pathology, one that possibly includes the small size of the brain or details of its endocast morphology. For myself, I think there is sufficient evidence to question whether LB1 is characteristic of its population.

Now we have additional evidence of body size in the population, including the two very short elements described above. Is that enough to believe that the brain is characteristic of the population? If there has ever been a case to invoke the "extraordinary claims require extraordinary evidence" clause, it is this one. LB1 is not only small-brained for a human, it is small-brained for any hominid.

So I don't think that pathology is a magic wand that is going to make this population into ordinary modern humans. But I'm not ready to jump to the conclusion that this was a population of hominids with australopithecine-sized brains. There is, after all, the problem of how they got to that island in the first place.

Speaking of getting to the island, what about their technology?

I think the tools are a complete red herring. There is every reason to think that modern humans were on Flores throughout the Liang Bua sequence. After all, modern people were on Australia by 50,000 years ago, and out to New Britain by 35,000. Maybe they bypassed Flores on the way, but it seems more likely that it would have been occupied long before these more far-flung locations.

Therefore, it is simplest to assume that modern humans made the tools and hunted the stegodon. Maybe they hunted the hobbits. Maybe some of the bones at the site are modern humans. Maybe some of them were dwarf modern humans.

Seems all tangled up, doesn't it? Yet the behavior speaks to the presence of Homo, and from the character of the tools, modern human seems likely. If those modern humans weren't the hobbits, then they lived alongside the hobbits.

So what's next?

We should see before long at least two (and possibly more) papers that dispute the Homo floresiensis interpretation. Carl Zimmer reports at his weblog that one of these will come from Robert Martin:

"Regardless of one's stand on this issue," Dr. Martin wrote to me in an email, "it is about time that the message got out that there are serious grounds for doubt about current interpretation of the Flores remains."

I think that's right. Of course Martin's interest is allometry of the brain, which made the initial interpretation -- island dwarfing of an early Homo variant -- seem very unlikely from the start. An australopithecine origin is less problematic from the point of view of allometry, but introduces the problems of biogeography -- how did they get there, and why weren't they anywhere else? Pathology would help a lot to explain that brain....

The pathology work will be the most interesting; I know there are many people working very hard to find a single pathology explanation that is consistent with the anatomy of LB1. If they have succeeded (and we should find out before long) it will be a major accomplishment.

And there will be comparative anatomical and possibly genetic work on pygmy people in the region. Remember the Rampasasa Pygmy Somatology Expedition? It will be coming to a journal near you.

The stealth factor is whether Max Planck (or anybody else) has gotten any DNA out of the bones. Wouldn't it be interesting if part of the mtDNA sequence looked like one of the more ancient human-specific nuclear genome mtDNA inserts (numts)? If this is an australopithecine population, mtDNA would be enough to show it.

More information here

All the Flores files

Original discovery

Endocast study

The damage to the specimens

Myth of the ebu gogo

References:

Brown P, Sutikna T, Morwood MJ, Soejono RP, Jatmiko, Saptomo EW, Due RA. 2004. A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia. Nature 431:1055-1061.

Falk D, Hildebolt C, Smith K, Morwood MJ, Sutikna T, Brown P, Jatmiko, Saptomo EW, Brunsden B, Prior F. 2005. The brain of LB1, Homo floresiensis. Science 308:242-245. Full text (subscription)

Morwood MJ, Brown P, Jatmiko, Sutikna T, Saptomo EW, Westaway KE, Due RA, Roberts RG, Maeda T, Wasisto S, Djubiantono T. 2005. Further evidence for small-bodied hominins from the Late Pleistocene of Flores, Indonesia. Nature 437:1012-1017.

If "short people got no reason to live", then why exactly do they live longer than tall people?

This study (abstract) from 2003 reviewed evidence for the relationship between stature and longevity, finding that short people have a substantial record of living longer in many large-population and some more select samples:

Findings based on millions of deaths suggest that shorter, smaller bodies have lower death rates and fewer diet-related chronic diseases, especially past middle age. Shorter people also appear to have longer average lifespans. The authors suggest that the differences in longevity between the sexes is due to their height differences because men average about 8.0% taller than women and have a 7.9% lower life expectancy at birth. Animal experiments also show that smaller animals within the same species generally live longer.

The paper is one of those reviews of dozens of studies, so there are lots of references. Here's an interesting factoid:

In addition, centenarians tend to be quite short and light [14, 63 and 69]. Japanese centenarians averaged about 10 cm shorter than 75 year olds and Hungarian centenarians averaged 154 cm. In an unpublished analysis, we compared 14 European countries divided into taller and shorter halves based on heights during youthful years and found the shorter countries averaged 77 centenarians per million vs 48 per million for the taller half. Short Sardinians and Okinawans, not included in this analysis, also have exceptionally high percentages of centenarians (136 and 340 per million respectively).

They also did a survey of famous dead people to see if the taller ones died sooner (they did). I wonder if that included the relatively short and long-lived John Quincy Adams, incidentally, the first president to wear long pants instead of knee britches (!). In fact, looking over the numbers, the dividing point for "short" famous people is 173 cm, or 5 feet 8 inches, which is a bit taller than average height for men.

The authors also suggest that the added longevity due to caloric restriction in experimental animals may actually be a reflection of small body size, rather than of caloric restriction per se. Their preferred explanation is called the "entropy theory" of aging, which essentially is the argument that the bigger you are, the more things can go wrong (Samaras 1974).

My major complaint is that more of the original data are not plotted -- there are a lot of plots of group means. But without seeing the variance in the data, it is impossible to tell if the effect of size comes from a large effect on a subset of very tall people, or a broad regression of longevity against size. In fact, I would strongly suspect that there is some optimum height that has the longest lifespan, with shorter lifespans on either side. This is almost certainly true, since congenital dwarfs do not have as long an average lifespan as nondwarfs. But the issue is where the optimum size may be -- is it relatively high within the range of, say, 5' 5" to 5' 8"? Or is it low, say, 4' 6" to 4' 10"?

Evolution of height?

Of course, this is one of those basic size questions that has a lot of impact on human evolution. For one thing, people today really vary in height in different populations. Do relatively tall populations pay a cost in longevity. The study suggests that at least in developed societies, they do. But how do pygmies compare? This kind of question may not currently be answerable, considering that differences in mortality rates in human populations owe much more to infectious agents than to anything else.

Rephrase the question, then: considering that mortality rates during recent human evolution were much higher (and average lifespan much shorter) than today, do the height differences in longevity that we now observe have any evolutionary relevance at all? It is hard to argue that their effect should have been the same as today. But on the other hand, it is hard to believe that radical size differences between populations had no effect on life history evolution (and longevity) at all. The question is how much effect, and how relevant the example of recent human demography.

Then there are the body size changes in the fossil record. Conventionally, we think that the increase in body size from Homo habilis (or a like-sized variant of early Homo) to early humans was accompanied by a substantial increase in body size (evidenced in particular by early human fossils like Nariokotome [KNM-WT 15000], KNM-ER 3228, and KNM-ER 1808). Conventionally, we also tend to think that lifespan increased at this time, along with body size. But the negative relation of body size on longevity in living humans (and possibly other mammals) creates an evolutionary challenge: if both longevity and body size increased (and substantially so in both instances), then some of the genetic changes accompanying this shift must have been under strong antagonistic selection -- pressure to find ways to eke greater lifespan out of larger animals.

Or how about Neandertals and later Europeans? The Neandertals were not taller, but they were bigger. Which is more important? The evidence suggests that Upper Paleolithic people had much higher lifespans than Neandertals. Could this have been a consequence of their smaller bodies? Or did it occur in spite of their greater height? And did the non-European contemporaries of Neandertals, many of whom were presumably smaller than Neandertals, possibly live longer? If so, such a life history difference may have been fundamental to the evolution of modern humans.

Lots of interesting possibilities. It is unclear right now to me whether the size of the size-longevity effect is great enough to account for many of these things (or in the case of early Homo, to affect it markedly). But an 8 percent difference in both size and longevity between men and women does indicate that the effect may well have been on the scale of evolutionary importance.

References:

Samaras TT. 1974. The law of entropy and the aging process. Hum Develop 17:314-320. Abstract

Samaras TT, Elrick H, Storms LH. 2003. Is height related to longevity? Life Sci 72:1781-1802. PubMed

Michelle Drapeau and colleagues (2005) report on the AL 438-1 specimen from Hadar. The specimen consists of "part of the mandible, a frontal bone fragment, a complete left ulna, two second metacarpals, one third metacarpal, plus parts of the clavicle, humerus, radius, and right ulna" (1). At 3 million years, the specimen is one of the youngest of the A. afarensis sample.

The AL 438-1 individual was evidently relatively large compared to the rest of the Hadar sample. The ulna length is 278 mm, which is larger than the mean for any of the human samples examined by Aiello et al. (1999) in their comparative study of the OH 36 ulna. It is about average for a chimpanzee, although chimpanzees have relatively longer forelimbs than would have been true of A. afarensis, so again this is evidence of a relatively large body size.

Drapeau et al. (2005) make a point of the proportion of the ulna and the mandible being similar to that found in AL 288-1 (Lucy), which they take as evidence that large teeth in this late specimen may be attributed to larger body size rather than greater megadonty:

Both Australopithecus afarensis mandibles have a larger corpus (breadth and height at M₁ relative to the ulnar size surrogate than those of African apes. Similarly, mandibular corpus shape (breadth/height x 100 at M₁) is similar in the two fossils (A.L. 288-1, 57%; A.L. 438-1, 60%). This difference in mandibular size corresponds to what would be expected from two extant ape conspecifics with ulnae of such different sizes. Since there are no differences between the two Hadar skeletons in mandibular to ulnar proportions, there is no evidence for an increase in mandibular size relative to the rest of the skeleton between the points in time represented by these two individuals. We cautiously offer this as support for Lockwood et al.'s (2000) suspicion that the observed temporal trend toward larger mandible size reflects a body size increase late in the Hadar time span of A. afarensis (Drapeau et al. 2005:41-42).

The paper has a substantial discussion of the morphology and comparative anatomy of the ulna. The bottom line of this analysis is that the ulna is similar to that of AL 288-1 in most respects, except for its larger size and somewhat greater curvature. It is, however, smaller and somewhat less curved than the later Omo L40-19, and substantially less curved than the OH 36 ulna. The authors write this about its similarities to other homionids:

While phenetically A.L. 438-1 presents a mix of ape-like and human-like morphology, when considered in a phylogenetic context, the Australopithecus afarensis forelimb shares synapomorphies exclusively with humans among extant hominoids taxa. It resembles non-hominins only in plesiomorphic character states. In this context, it is apparent that A. afarensis forelimb anatomy reveals the results of selection for a more human-like humeroulnar joint, larger thumbs, and altered carpometacarpal joints that reflect an emphasis on manipulative aptitude at the expense of forelimb-dominated climbing ability (Drapeau et al. 2005:43).

That is a relatively powerful statement of the adaptive qualities of the A. afarensis forelimb, which appers more or less necessary to explain the differences between early hominids and apes in this respect. If the early hominids were really climbing a substantial proportion of the time, then we might hypothesize that their forelimbs ought to look more like ape arms. But they don't; there are clear differences that make the early hominid arms look more similar to human arms. Thus, the authors turn to the hypothesis that the A. afarensis forelimb is additionally adapted to "an emphasis on manipulative aptitude."

At the moment, this hypothesis remains to be strongly tested. Most of the human-like features of the A. afarensis arm are arguably the result of not being used in quadrupedal weight support. Thus, the fact that "the Australopithecus afarensis elbow joint appears to reflect habitual loading the elbow at or near 90 degrees, rather than optimization for loading in a more extended posture as in extant apes" (43-44), as well as the anatomy of the joint and the form of the carpometacarpal joints may all be explained by the fact that early hominids were not knuckle (or fist) walkers. The large thumbs are the strongest piece of evidence for any kind of manipulative behavior in A. afarensis.

On the subject of retained similarities with apes, Drapeau et al. (2005:46) have this to say:

The retention of African ape symplesiomorphies in A. afarensis may be attributed to either stabilizing selection fore a partially arboreal locomotor repertoire, or to lack of selection against these traits (see discussion in Stern, 2000; Ward, 2002). It is inherently difficult to test these alternative hypotheses. Thus, the significance of these retained traits for reconstructing the behavior of A. afarensis is difficult to determine with certainty in the context of demonstrable selection for a human-like elbow and hand joints. Australopithecus afarensis shares some apomorphies with humans that suggest emphasis on use of the forelimb in flexed postures, and improved grip capability relative to apes. The presence of these synapomorphies suggests similarities in forelimb function among hominins, likely reflecting selection for expanded manipulative capabilities and flexed forearm postures relative to that found in apes and a diminished capacity for ape-like arboreal behaviors. Only later did humans display evidence of further selection for manipulation coupled with reduced forelimb robusticity. We conclude that in Australopithecus afarensis, selection for natural manipulation outweighed selection for arboreal activities, but that selection for refined manipulative ability had not yet come into play in human evolution.

A fine balance, if it is true, and fitting within the generally understood picture that, with regard to its arm and hand functions, A. afarensis was either Homo habilis nor a chimpanzee.

References:

Aiello LC, Wood B, Key C, and Lewis M. 1999. Morphological and taxonomic affinities of the Olduvai Ulna (OH 36). Am J Phys Anthropol 109:89-110.

Drapeau MSM, Ward CV, Kimbel WH, Johanson DC, and Rak Y. 2005. Associated cranial and forelimb remains attributed to Australopithecus afarensis from Hadar, Ethiopia. J Hum Evol Advance before print.

Lockwood CA, Kimbel WH and Johanson DC. 2000. Temporal trends and metric variation in the mandibles and dentition of Australopithecus afarensis. J Hum Evol 39:23-55.