Notable paper: Aubert M, Brumm A, Ramli M, Sutikna T, Saptomo EW, Hakim B, Morwood MJ, van den Burgh GD, Kinsley L, Dosseto A. (2014). Pleistocene cave art from Sulawesi, Indonesia. Nature 514:223-227. doi:10.1038/nature13422
Synopsis: The paper is mostly devoted to providing date estimates for the formation of speleothems, in this case "cave popcorn" deposits of calcite that have formed on surfaces that bear rock art. The authors show that many examples of rock art are Paleolithic in age, and paintings from at least three sites are more than 35,000 years old -- making these older than Chauvet Cave in France, the oldest example of figurative art in Europe. The oldest date for the Sulawesi rock art is 46,000 years ago, from Leang Barugayya 2.
Important because: The paper provides a second dot on the map of representational art before 30,000 years ago.The rock art from this area of Sulawesi was first noted by prehistorians more than 50 years ago. In southwestern Europe, rock art specialists have worked hard to develop a chronology of cave art based on chronology, but really that effort depends on assumptions about cultural groups that we cannot generally justify -- and rock art far from Europe could never fit into a European chronology. The appearance of new methods of dating the overlying speleothems has helped to open the science.
A clever article by Rose Eveleth for The Atlantic looks into a peculiar regularity in the history of art: "Nobody Knows What Running Looks Like". Eveleth reviews the findings of a recent paper by Julian Meltzer, which investigates representations of human gait throughout the history of art.
Normal human gait has the arms swinging opposite the legs. A step forward with the right leg is accompanied by a swing forward with the left arm. This arm swing facilitates a counter-rotation of the trunk and overall conservation of energy, and is natural for most people as soon as they learn to walk.
Strangely, most artists who depicted humans running or walking have gotten it backward: showing people swinging the same arm and leg forward, not opposites. Eveleth shows some great examples from ancient Egypt up to the present day.
When it comes to art, it’s possible that rather than being an error, the awkward, one sided lurch forward is an artistic choice. In Egyptian art, for example, artists followed strict rules about the position of the head and body. But as art evolved, and as accuracy of form become more and more important, it’s hard to imagine why someone like Da Vinci or Donatello would intentionally draw a person running in such an inaccurate pose. And modern “how to draw” guides are certainly not intentionally teaching someone the wrong posture.
A survey of people shows that they report this aspect of gait significantly worse than chance...
And when participants were asked to pose in mid-run, only 14 percent of them picked the pose that actually reflected running. The other 86 percent froze with the same sided arm and leg moving forward.
The investigation into this common error leads to some interesting conclusions about how we learn to move.
Meltzoff J. (2014). Errors in the making and perception of art images of human gait: Psychological explanations. Psychology of Aesthetics, Creativity, and the Arts 8(3):321-329. doi:10.1037/a0036669
The Mail and Guardian has a nice article about the work of John Gurche, written by Sarah Wild: "The next best thing to a time machine". Gurche is doing a guest lecture at the University of the Witwatersrand this week, and the article reviews his work and the importance of paleoartistic reconstruction to the science of paleontology:
“The world – worlds? – that science reveals is breathtaking, full of wonder. But the language of science is not accessible to many people. To convey that wonder, or even just to create an image that communicates what the scientific literature is saying, you need art,” he says, arguing that this is an example of science serving art. “Science can feel like a muse to the artist.”
But in this case, the muse is a collection of bones, painstakingly excavated from the ground over the course of years. In the coffee room in the Bernard Price Institute at Wits, the researchers have a puzzle on the table to help them to sharpen their visual abilities for piecing together ancient fossils.
When it comes to palaeoscience, these researchers have to put together a puzzle without knowing much of the picture in advance, and Gurche in effect has to colour it in.
I've seen skilled anatomists spend an awful lot of time with those jigsaw puzzles in the tea room.
The collaboration between artists and anatomists is so important to both fields of study. The best artists share with anatomists a skill of vision, developed over thousands of hours of close study, that is impossible to describe with words.
Gurche's recent book, Shaping Humanity: How Science, Art, and Imagination Help Us Understand Our Origins, provides a deep perspective on the creation of his distinct visions of ancient human ancestors.
ScienceOnline has now ended forever. Many people in the next several weeks will doubtless offer their perspective on why the organization failed. They will point to a series of poor decisions, a tone-deaf response to a sexual harassment scandal, and an organization's alienation of thoughtful community members.
I'd like to consider for a moment why it succeeded so brilliantly for more than five years. ScienceOnline represented a new wave of fusion between scientists and culture. That spirit has been taken outward into many directions by many of the participants, including me. The conferences provided a nexus for people from all walks of science to experience a changing landscape of sharing and doing science.
Here's a list of six things that I think helped to make ScienceOnline work.
Youth. The fascination of science is not obvious to everyone, and science has always relied on talented interlocutors to translate important results into popular culture. From the 1990s to the mid-2000s, many of the most widely influential science communicators -- journalists and high-ranking scientists -- were old, crusty, and hidebound. Online culture was new, young and full of revolutionary and romantic ideas. In science, where the most senior ranks are very white and very male, youth also brings diversity. The participants in ScienceOnline were well placed to write and speak about science in new ways, which was exciting to people.
Community. ScienceOnline created a forum in which online friends could meet. More important, the conference enabled people to meet who didn't know each other's online work. Many participants didn't even have online work, in the sense of a blog or other regular writing presence. The conference was built around their mutual interests in new forms of doing, publishing and communicating science. They already formed an informal network of people across institutions, disciplines, and professional callings -- from students to writers to full-time scientists. ScienceOnline helped to solidify this network and enable its members to call upon each other's skills outside of the usual professional silos.
Maker culture. People who engage with science and the internet are creators. They make things, grapple with new technology, hack the source. ScienceOnline gave these people a place to share what they were making. Artwork, ebooks, video, websites, software -- even new startups like Mendeley -- were all shared at ScienceOnline. The conference was full of "how-to" sessions, focusing on the practical side of making things. Some were beginner sessions that helped people learn how to blog, how to manage intellectual property, how to be a better writer. One year, I even organized a nuts-and-bolts workshop on how to migrate to a new server. Where else could one go from a session on documentary film techniques to a session on e-book platforms, and have both of them hit the right level? It is just fun to be around so many different people who are making different kinds of things.
Populism. Science is hierarchical. Senior scientists get the most resources and are allocated the most attention at conferences. At ScienceOnline, students, upcoming writers and journalists, and more established scientists and other academics all collaborated to present the program, all attended the same events and sessions, and most importantly all interacted on an equal footing. The conference encouraged sessions in which the attendees were active participants, not a passive audience. Much has been written about the deficiencies of the "unconference" format, in particular how relying on the crowd of attendees can miss valuable expert knowledge. But giving a broader sample of attendees a voice enabled them to help create knowledge instead of passively receiving it.
The Waiting List. Let's face it, nothing is cooler than the nightclub with a line out the door. At its peak, the tickets for the ScienceOnline conferences were snapped up within minutes of registration opening. In those years, more than a hundred potential attendees might get on a list to wait for cancellations. The organization did something very smart with this popularity: It gave away spaces to people who were willing to propose and organize great sessions. That idea fueled a diverse program with highly motivated presenters and facilitators. These people helped to minimize the inherent disadvantages of the "unconference" format, preparing and focusing material that would be engaging to the participants. As the organization started to falter, it neglected the value of session organizers increasingly discarded this strength, devaluing the contributions of session organizers.
The Backchannel. ScienceOnline was not the first conference with an active Twitter backchannel, nor was it the first to have livestreaming of sessions. But it was the conference that made offchannel more important than onchannel. What other conference had bandwidth demands on the scale of the Superbowl? The video streams enabled people offsite to participate virtually in the meeting through Twitter, reacting to sessions with their blogs, and thinking about how to organize similar sessions at other meetings.
Many of the great things about ScienceOnline have spread across the culture of scientific meetings. Those of us who tweet from scientific meetings have developed a backchannel bond, almost a shadow conference going on behind the scenes. Other meetings may not be egalitarian unconferences, but at least the senior scientists don't seem to grandstand during the question period quite so much as they used to. Some of today's best-known and most talented science journalists are people who met and learned from each other at ScienceOnline. Those relationships continue to shape the landscape of science communication.
Many of these changes may have been more or less inevitable, as a wave of technology enabled new forms of interaction among younger scientists and journalists. ScienceOnline rode this technology wave, capitalizing on the interests of people adopting new modes of writing and interacting. But it rode the wave in style, actively encouraging or driving some changes, with so many of its attendees involved across disciplines in other areas of science and culture.
The next ScienceOnline, whatever it may be called, will have to catch a different wave. The original never really evolved beyond the time when blogs were new. Blogs were quick and nimble mainly because they were small, with few layers of editorial decision-making. This gave them an unmatched power to respond to events, but has limited their reach. The leaders of online science have gone beyond the blog, exploring many other avenues of creativity. Already, online technologies are making an impact on documentary filmmaking, education, granting agencies, scientific publishing and field research. Interactive video, MOOCs, massive Facebook communities, and live-online hybrid events have pushed the boundaries of science communication in new ways. We are still making stuff -- even knocking on the door of that most hidebound mode of communication, the television news. With these successes, the makers have become leaders in their fields, spreading new approaches and techniques into more specialized communities. Tomorrow's online science conversations must look forward to this diverse future.
Nature last week carried a great article by Barbara Fraser about the growing research into the earliest peoples of South America: "The first South Americans: Extreme living". She followed researchers Kurt Rademaker and César Méndez on their respective fieldwork projects in Peru and Chile.
The landscape looks bleak, but Rademaker views it through the eyes of the people who built a fire in the rock shelter, named Cuncaicha, about 12,400 years ago. These hunter-gatherers were some of the earliest known residents of South America and they chose to live at this extreme altitude — higher than any Ice Age encampment found thus far in the New World. Despite the thin air and sub-freezing night-time temperatures, this plain would have seemed a hospitable neighbourhood to those people, says Rademaker, an archaeologist at the University of Maine in Orono.
“The basin has fresh water, camelids, stone for toolmaking, combustible fuel for fires and rock shelters for living in,” he says. “Basically, everything you need to live is here. This is one of the richest basins I've seen, and it probably was then, too.”
To me, the kind of work being done in South America and in Mexico upon early Americans is really inspiring. Archaeologists are finding many sites that document the human presence in the Americas before 10,000 years ago, including provocative skeletal remains from deep inside of Yucatan caves. The situation in the U.S. and Canada has also changed with the addition of new sites and methods (including paleogenetics). We are at the borderline of a revolution in our approach to the initial habitation of the Americas.
Gorham's Cave is on the eastern face of Gibraltar. It was carved by the sea, and now lies above sea level because of the upthrust of the Rock over hundreds of thousands of years.
I took this photo from a boat offshore, and the one thing it lacks is a convenient scale. The cave entrance soars more than forty meters high.
The Scientist has a nice article about the evolution of trichromatic vision in primates: "The Rainbow Connection". Trichromacy in anthropoid primates is a great example of many evolutionary mechanisms, and I discuss it every semester in my introductory course. New World monkeys have a different mechanism (allelic trichromacy) than catarrhines, and the appearance of the two kinds of trichromacy is a clear example of evolutionary convergence.
The linked article discusses some interesting consequences of allelic trichromacy, which leaves most New World monkey populations polymorphic for color vision -- a mix of heterozygous individuals who are trichromats with homozygous individuals who are dichromats:
Amanda Melin of Washington University in St. Louis has spent years traveling to the forests of Costa Rica to observe capuchin monkeys in their natural habitat. Like squirrel monkeys, these New World primates have dichromatic males and either dichromatic or trichromatic females. Melin spends days at a time watching the animals forage, walking kilometers through the forest as the monkeys move from tree to tree, and collecting DNA from fecal samples, in an effort to determine which colors each animal can see. Much to her surprise, she’s found that fruit feeding rates between dichromats and trichromats are the same, initially suggesting that color vision doesn’t offer an advantage for foraging.
Digging deeper into the data, however, Melin uncovered a subtler effect. “Where we see the difference is in accuracy,” she says. “Trichromats are making way fewer mistakes, but foraging at a more leisurely pace.” Dichromats, on the other hand, appear more frantic, touching, sniffing, and biting more fruits, including unripe or inedible ones. The question Melin is trying to answer now is whether that sloppier foraging behavior has any nutritional impact on the animals.
I also love the accompanying infographic, which emphasizes that many kinds of vertebrates have even better vision than anthropoid primates. Everything from goldfish to chickens see four colors, with sensitivity into the ultraviolet!
The alumni magazine of the University of Wisconsin-Madison has done a great article about my recent massive open online course (MOOC), written by the science writer Jill Sakai: "Behind the Screens". The article introduces some of the team behind the creation of the course, and discusses how we made some of the course's more unique elements:
He assembled a portable video kit and carried it everywhere, recording interviews with experts and lectures at field sites all around the world. He caught colleagues at conferences and at digs, in labs and over beers, and he talked to them about what they do and why, what they’ve learned, and what it means. He sent small cameras into the field with UW students to capture the unique feel of an archaeological site — a mixture of tedium and toil punctuated by flashes of discovery.
Then, working with a team from the academic technology group of the university’s Division of Information Technology (DoIT), he edited dozens of hours of footage into eight units of five- to twenty-minute video segments that were posted on the Coursera site each week from mid-January through mid-March this year.
I have been following up the data we gathered during the course. We saw some remarkable outcomes -- including the global scope of the students, their willingness to assist in evaluating their own learning during the course, and their interactions with each other through discussion forums. I have been inspired by some of the stories students told about their experiences. The outcomes have deepened my commitment to bring the stories of human evolution research out to a global public.
The entire process was enormously time-intensive, and the graduate student TA's were incredibly important to the success of the project. I'm very pleased that Sakai was able to feature the work of the entire team:
In the sea of students, those forums served as a source of community, say graduate teaching assistants Sarah Traynor PhDx’15 and Alia Gurtov MS’13, PhDx’15, with posts ranging from the serious to the lighthearted. “They made a forum called The Pub, and people would go in and chat, [writing], ‘Welcome to the course, grab a beer, and sit down and talk!’ ” says Traynor.
Watching the videos is less like watching documentaries and more like sitting in on real conversations between people who happen to be world-class experts and professional colleagues. Their talks dig into the very roots of the human race.
As many readers have noticed, I have begun releasing some of the videos here on the weblog and through YouTube. I'll continue to premiere these during the next few months, particularly as I have been adding new interviews and films in anticipation of offering the course again sometime in 2015.
Notable paper: Shipton, Ceri, Ash Parton, Paul Breeze, Richard Jennings, Huw S. Groucutt, Tom S. White, Nicholas Drake, Remy Crassard, Abdullah Alsharekh, and Michael D. Petraglia (2014) Large Flake Acheulean in the Nefud Desert of Northern Arabia. PaleoAnthropology 2014:446-462. doi:10.4207/PA.2014.ART85
Synopsis: The authors are part of the University of Oxford Palaeodeserts Project, and they report on the results of survey work in the Nefud Desert of Northern Arabia. They located four sites with Acheulean bifaces, documenting human occupations during wetter climatic phases when large lakes formed in the northern part of the Arabian Peninsula.
Important because: Although later sites representing Middle Paleolithic industries are known from northern Arabia, the earlier time period has so far been represented mainly from the coastal fringe and the Hadramaut. Northern Arabia helps to fill a handax-sized hole between the well-known sites of the Levant and those of India.
But... It is misleading to cast the archaeological problem as one of routes of dispersal to India. There were hundreds of thousands of years at play, small hunter-gatherer bands fluctuating along with the local climates. The comparatively well-watered Levant and Mesopotamia were always connected through Syria and southern Turkey -- the "fertile crescent". That makes northern Arabia a similar case to much of the Sahara, an area that nearby human populations may have raced to exploit during the occasional time intervals when rainfall was higher. A serious area of pulsed exploitation outside of Africa may have figured into gene flow or dispersal through much of Asia, without being a "connection" between geographic regions.
This summer I pointed to an article about the FwJj20 locality at Koobi Fora, which provides the earliest known evidence of systematic fish exploitation in the archaeological record ("The fish of Koobi Fora"). I mentioned that another interesting paper in the journal was worth discussion, by José Joordens and colleagues, titled "A fish is not a fish: Patterns in fatty acid composition of aquatic food may have had implications for hominin evolution".
The article takes a nutritional ecology approach, examining the empirical question of whether African lake fish have similar nutrients as Indian Ocean marine fish, focusing particularly on fatty acids, DHA and EPA. They find some differences between the oceanic and lacustrine fish in these resources, but focus most of their essay upon a review of the literature on the nutritional value of fish in freshwater and marine environments and their possible importance in human evolution.
There are many good things in this review. I have written about aquatic resource use many times before, including Joordens' work. I think that aquatic resource use shows something very important about the behavioral and dietary flexibility of early Homo.
I commented in 2009 on the FwJj20 locality ("Fishy story from Koobi Fora"). , emphasizing that the demonstration of aquatic resource use is very different from showing that a lack of aquatic resource use is a
I accept that reptile and fish meat may be nutritionally desirable. The question is whether they caused the increase in brain size associated with Homo. One way to read that hypothesis is as Lamarckism, which is simply wrong (Larry Moran has commented on that topic). I don't think that any paleoanthropologists are seriously Lamarckist, but some need to be more careful how they describe the relationship of fitness and diet.
Joordens and colleagues provide an important corrective to some of the earlier claims about aquatic resources. In doing so, they bring the comparative method to the issue of aquatic resources:
Further, we feel that it is essential to distinguish between a triggering, facilitating and/or driving role of aquatic resource exploitation. If aquatic resource exploitation had been the major driver for hominin brain expansion, we would expect that this mechanism would have led to a similar enormous brain expansion in other animal lineages such as carnivores foraging in aquatic ecosystems. Indeed, considerable brain expansion has occurred in cetaceans (particularly in Delphinidae), but it took place millions of years after adaptation to a completely aquatic lifestyle and could not be causally connected to an aquatic lifestyle (Marino, 2007 and Xu et al., 2012). In extant herpestids, mustelids and procyonids, the species that eat aquatic prey have the largest absolute brain sizes of their families (Shabel, 2010), but these increases in brain size do not reflect an explosive brain expansion and increase in cognition such as is seen in the human lineage (e.g., Roth and Dicke, 2005). This suggests that while aquatic resource exploitation by hominins may have triggered and/or contributed to initial moderate brain growth, one or more additional factors are required to explain the extreme events in brain growth and cognition in the hominin lineage.
This is a very important perspective on brain evolution. Certainly, larger brains have nutritional requirements. But did particular foods "trigger" the evolution of the brain, did they "facilitate" brain evolution, did they "drive" brain evolution? Were dietary changes necessary for brain evolution, or sufficient for it? Or neither?
Taxonomic breadth shows behavioral flexibility. In my opinion, dietary flexibility is a more important adaptive change than fish nutrition alone could be. Again, from my 2009 post:
Aquatic animals aren't important because of their sheer numbers, but because they tell us about the flexibility of foraging behavior. Living hunter-gatherers eat turtles and reptiles when they can, and because they are usually small food packages, they often eat them where they find them instead of returning to a base camp first. Hunter-gatherers are flexible in what they eat and where they eat it. FwJj20 is showing at least a substantial taxonomic flexibility in the meat-eating of early Oldowan hunters.
Too many scientists focus only on the "supply side" of brain energetics, emphasizing that a high-quality food source with abundant protein and fat is necessary to grow and fuel a brain. All other things being equal, an increase in the supply of food will reduce the force of selection on brain size. Understanding an evolutionary change in a trait requires us to examine the correlation between that trait and survival or reproduction.
Stone tool manufacture and tool-aided meat exploitation, starting by 2.6 million years ago, predate any substantial evidence for brain expansion by a half million years or more. Brain size hopped from an australopith value less than 500 ml up to around 600 ml in Homo habilis and early Homo erectus, and up to around 800 ml in later Homo erectus. Those changes have no obvious archaeological correlate. The later, massive increases up to a human brain size of 1350 ml or higher also have no obvious archaeological correlates -- but more important, as Joordens and colleagues point out, they have no obvious dietary correlates.
The literature on aquatic resource use, with exceptions like Joordens and colleagues' work, seems to assume that all of human evolution happened on East African lakeshores. Sure, the hominin assemblages of Kenya, Tanzania, and Ethiopia are embedded in sedimentary layers that represent river, marsh or lakeshore deposits. But nobody's talking about the giant catfish eaten by early Homo at Swartkrans because there aren't any. Looking more broadly, occurrences of Early Pleistocene Homo stretch from South Africa to Spain to China and Java, and include some instances with good evidence of aquatic resource use (Trinil, Gesher Benot Ya'aqov, El Kherba) and some without (Dmanisi, Sima de Elefante, Drimolen).
We have barely begun to sample other parts of Africa that lack the long lacustrine sedimentary exposures of the Rift Valley. Homo erectus or species substantially like it had a widespread distribution after 1.8 million years ago, and were essentially unknown before that time. The first occurrence of H. erectus may be Dmanisi, or Swartkrans, or even Mojokerto -- any two of which represent a greater ecological and geographic distance than separates the entirety of the hominin fossil record before 1.9 million years ago. We have a lot of searching to do.
Joordens, JCA, Kuipers RS, Wanink JH, Muskiet FAJ. (2014) A fish is not a fish: Patterns in fatty acid composition of aquatic food may have had implications for hominin evolution. Journal of Human Evolution doi:10.1016/j.jhevol.2014.04.004
Driving up the Rock of Gibraltar last month, to a dinner in St. Michael's Cave, we were treated to this sunset view, westward over Algeciras, Spain:
Jeff Leach, at the "Human Food Project", has written pungently about a bout of microbiome self-experimentation: "(Re)Becoming Human: what happened the day I replaced 99% of the genes in my body with that of a hunter-gatherer".
AS THE SUN set over Lake Eyasi in Tanzania, nearly thirty minutes had passed since I had inserted a turkey baster into my bum and injected the feces of a Hadza man – a member of one of the last remaining hunter-gatherers tribes in the world – into the nether regions of my distal colon. I struggled to keep my legs in the air with my toes pointing towards what I thought was the faint outline of the Southern Cross rising in the evening sky. With my hands under my hips – and butt perched against a large rock for support – I peddled an imaginary upside down bicycle in the air to pass the time as I struggled to make sure my new gut ecosystem stayed put inside me.
I have to say, this is just wrong.
For one thing, many people around the world carry potentially harmful parasites, from tapeworms to treponema. Shooting these up your anus is a bad idea. Leach admits later in the post that he blithely ignored this risk:
However, I had no data on the parasites he might carrying at the time of the transplant as those analysis were still ongoing at Jack Gilbert’s lab at the University of Chicago. Oh well, parasites be damned, onward with the science!
For another thing, the Hadza have their own long evolutionary history. Their diet is merely one representative of the marked dietary diversity of recent hunter-gatherers. Other foraging groups, for example, the Ache of Paraguay, have a very different dietary composition. The study of these microbiomes is scientifically very interesting, and we may discover commonalities among them. But the idea that the microbiome of any Hadza person represents an "ancestral" or "healthy" human population is nonsense. They have their own distinctive set of challenges affecting their microbiomes, including the aforementioned parasites. A microbial community that has formed within a Hadza gut might work equally well anywhere else, but there's really no reason to expect that it will.
Scientists are working to establish which aspects of microbial communities may be associated with health outcomes, and an important part of that process is understanding how human populations vary. Westernization of diet is one of the major influences on microbiomes, and studying forager microbiomes is a good idea. Fecal transplants have proven useful in cases of recurrent infection by pathogenic bacteria, and it is possible to imagine that intestinal health or obesity might reflect the action of the microbiome.
Still, there seems to be a lot of quackery here -- almost a New Age belief in the power of a gut unspoiled by burgers and fries.
A week ago I was in Gibraltar for the 2014 Calpe Conference. The conference this year is focused on issues of world heritage, as the Neandertal sites of Gorham's Cave and Vanguard Cave will be proposed for World Heritage status from UNESCO next year.
There are more than a thousand inscribed World Heritage Sites around the world, but only a tiny number of these are sites that represent human evolution. Of course I consider every Paleolithic site a piece of world heritage, and it is lamentable that only a handful have received this recognition from their governments. One of the remarkable things about my MOOC, Human Evolution: Past and Future, is that I was able to visit many of these World Heritage Sites and capture their importance on video for the students worldwide.
In honor of my visit to Gibraltar, I'll share a video that I made with Geraldine and Clive Finlayson, giving a first-hand tour of the archaeological levels in Vanguard and Gorham's Caves.
The video is one of three that I made with the Gibraltar Museum team for the MOOC. It was a great pleasure to visit the sites again with them, and now that the engraving in Gorham's Cave has been revealed to the public, it is an exciting time to revisit the Neandertal evidence from these sites.
I received a letter about lactase persistence that motivated me to a fairly long reply; I thought I would share the question and answer:
I read with interest your article in the current Scientific American (Still Evolving). I do have one serious puzzle: While not a biologist (or an anthropologist), my understanding of the logic of Darwinian selection is that mutations are random and that they only have a chance of sticking around if they give the species some benefit for survival in the context of the current environment; and that survival translates primarily into the capacity to produce enough healthy offspring to continue (and maybe sometimes produce more offspring than before).
Now, when it comes to protection, even moderate, against malaria, this makes sense. But I do not see how lactose tolerance would fall into that category. My impression is that humans who are lactose intolerant and drink milk do not appear to have a life-threatening reaction. They may be miserable at the extreme, but they live. Certainly, being lactose tolerant in a population that raised dairy cows (goats, etc.) would improve one’s quality of life, but I need some help in understanding how this would benefit our species survival ability.
This is a great question; I had a very prominent geneticist ask me the same thing some years ago.
It can be difficult for people in today's industrialized societies to perceive the depths of the nutritional challenges faced by most people throughout history -- sadly even though the same nutritional challenges are still faced many people living today. Nutritional shortfalls impede reproduction in many ways. Women on calorie-restricted diets have lower fertility, and take longer after the birth of a child to conceive again. Adolescent girls with restricted caloric intake take longer to reach sexual maturity.
The lactose in milk amounts to approximately a third of its caloric content. That means that a person with lactase persistence can get up to 50% additional energy from milk. Milk can be processed by microbes to reduce its lactase content -- many yogurts and cheeses can be consumed by adults who do not have a persistence of lactase. But these methods are time-consuming and reduce the amount of energy available from a given volume of milk.
In a milk-consuming population, otherwise nutritionally limited, lactase persistent women would have reproduced earlier and have had larger families. Our best estimate is that this advantage was on the order of 10% during the last few thousand years in northern Europe. This kind of advantage could accure by lactase-persistent women starting their reproductive lives around 2 years earlier, or spacing their children around three months closer together. That kind of change in birth spacing or maturation need not be genetic itself; it can emerge simply as a result of better nutrition.
Lactase persistence worked especially well in societies where it was part of a system. Milk is a very efficient use of domesticated animals; milking animals yields much more energy over the long run than raising them for meat. Some ancient populations relied extensively on the milk produced by their herds; this was a much more important component of the diet in areas that were too arid or too cold for the earliest domesticated grain crops. Those areas, like northern Europe, Arabia, and the Sahel, are where different lactase persistence mutations most took hold.
Once such a system got started, people who carried the original, non-digesting lactase allele would have been even worse off. Consuming moderate quantities of milk can lead to digestive distress in people who do not produce lactase. The resulting diarrhea reduces energy intake even more and can cause worse health problems. That side-effect of milk drinking might initially have deterred people from adopting milk in their diet. But for any group where milk drinking had been established already, the bad consequences of lactose intolerance would increase the selective pressure toward lactase persistence.
I've kept a post by Kristina Killgrove sitting on my desktop for a long time. Although the post is specifically about a particular highly-reported study on ancient Roman remains, I find some parts of it to be much more generally applicable: "Baby bones were trash to Romans". As a Roman scholar and bioarchaeologist, Killgrove writes against the conventional storyline when bones are found in contexts that would be unusual within our peculiarly funerary-obsessed culture:
The reporting of infant burials is always problematic to me, though. From the "brothel babies" to the Carthaginian tophet infants to these Etruscan neonates, the headline is always about how unfeeling people of the past were about babies. It's a longstanding trope - that death was just something people used to put up with, that they were hardened to its devastation - but anthropologically and historically, it's not usually based in fact. We simply like to tell ourselves that we're better than our forebears, that we're more civilized than the Etruscans/Romans/Carthaginians, that we've culturally evolved to do right by our biological progeny. But we do a disservice to the past by assuming a lack of emotion, and we do an even greater disservice when we over-interpret small amounts of data to arrive at those conclusions.
Bioarchaeologists work to characterize the social context of human remains from details like their health status and physical treatment after death. That social context varies culturally, different cultures with different rules and attitudes about the treatment of the dead. Those cultural differences do not necessarily imply differences in the emotions people feel; they are ways of channeling those emotions.