Hyena carrying away meat, trying to escape the other Serengeti scavengers.

3

Triple Startups about 2.5 Million Years Ago

In Africa, there were a lot of new species about 2.5 million years ago, when three big things happened:  stone toolmaking, the ice ages, and a hominid spinoff with a bigger brain.

            Climate had been gradually cooling and drying in the habitat of the upright ape.  This made the forest patchy where it had been continuous, with forest surviving only in particularly good soil or in the uplands that effectively tickled the passing rain clouds.  This subdivision meant that there was more forest edge than ever before, thus creating more woodland and more woodland apes.

            Between 3 and 2 million years ago, climate became chaotic.  Up north, the winter sea ice reached as far south as France 2.51 million years ago, which is as good a time as any other to say that the ice ages “began.”

            In Africa, the problem was not ice.  It stayed confined to the upper reaches of such places as Mount Kenya and Mount Kilimanjaro, visible on a clear day but not intruding into most hominid lives.  At sea level, the air temperature was only a few degrees cooler – more comfortable if anything.  The African problem was that less rain fell.  At times, the rain forests shrank by 80 percent.  Major East African lakes, even immense Lake Victoria, eventually dried up.

            Most of what we know about hominid evolution in this period comes from the limestone caverns of South Africa or the old lakeshores of the Rift Valley up north, especially a series of fossil sites ranging from Laetoli, 3 degrees south of the equator, to Hadar at 11 degrees to the north.  Early hominid fossils have also been discovered to the north of Lake Chad, a half-continent to the west of the Rift.  Forests are unlikely to preserve bones; our best views into hominid evolution occur at places where either sediments from a flooding lake bury a corpse, or where someone gets trapped in a cave and the stalactites drip on their bones, sealing them in stone.

            We have little choice but to sympathize with the drunk who searched for his lost keys under the streetlight, because the light was better there.  We don’t know where the lost bones of the earliest hominids are but have to satisfy ourselves with what we can find where preservation is better and, since no one has the money to dig deeply into the earth, in the present-day badlands where the surface erosion exposes the sediments of those ancient lakes.

            Looking for ancient tools follows pretty much the same imperatives, prospecting where the ancient lake edges were or the shelters nearby.  Much of the dating of these sediment layers comes from the easily dated volcanic layers, which often sandwich the artifact layers of interest.  The flora and fauna of a period in volcanic East Africa often allow the hominid fossils in the caves of South Africa to be dated, via those associated fauna that spread between East and South Africa.

            From such fortuitous circumstances come our occasional glimpses of the state of mind of our ancestors.  Could they innovate?  Could they postpone advantage (as in delaying consumption, in order to prepare the food better)?

            It is thought that diet became less vegetarian, with more high-quality food such as meat, about this time.  Some view eating meat as a setup for brain size increase, and it’s an interesting line of reasoning.  Gorillas have such long guts (and big bellies) because their leafy diet is low in quality, rather like “calorie free” zucchini.  So a long gut gives the food more of a chance to give up its calories to the intestinal wall.  This also means that the gut needs a lot more blood supply, just to service the long “disassembly line.”  If, so the reasoning goes, you improve the diet with meat (that is readily broken down and absorbed), the variants with shorter gut lengths would still get by.  Those variants with both shorter guts and bigger-than-average heads would then have the extra blood flow needed by the bigger brain.

            Some marsupials devote less than 1 percent of what their heart pumps to servicing their brains.  In the average mammal, it is about 3 percent.  In humans, it is close to 16 percent.  If something had to give, it was likely the overextended guts.  An ancestral diet of low quality food, on this argument, keeps bigger brained variants “vegetating” while digesting a meal.

            So the dietary shift into more meat, while not “causing” bigger brains, helps to clear the path for any bigger-brained variants that come along.  It doesn’t say anything about whether this meat was scavenged or hunted.  Given all the late-arriving lions, hyenas, jackals, and vultures, any hominid lucky enough to kill a grazing animal would shortly face some competition.  Defleshing using sharp tools leaves behind “cut marks” on bone, and such cut marks have been found as early as 2.5 million years ago in Ethiopia.  They are on the bones of large animals, even hippos – not the small monkeys that chimps and bonobos opportunistically hunt.

            Whether hunting or scavenging, it represents a major behavioral change because of dealing with the competition.  For these hominids, it wasn’t just a manner of avoiding becoming dinner for a lion but a problem of how do you chase them off or avoid them.  Tearing a piece loose, and making off with it, is a common strategy.  (You need to leave enough behind so that the competitors are more attracted by it than you.)  Hyenas can be seen to do this in Ngorongoro Crater, before the lions arrive to take the carcass away from the hyenas that made the kill.  (No, that’s not backwards; in the nearby Serengeti, it is true that the lions hunt and the hyenas primarily scavenge, but they switch roles in the crater highlands. In general, many scavengers can hunt and many hunters happily scavenge.)

            Making meat quickly portable might have been an early application for fractured stones with a sharp edge.  Bash one rock into another, and then search for an edge sharp enough to amputate a leg at the knee or hip.  (Indeed, just throw a rock against another rock and then search the fragments.  But aimed blows are less wasteful of raw materials than the primitive “shatter and search” technique.)  Then run with the limb and haul it up the nearest tree, allowing the lions and hyenas to be distracted by the rest of the carcass.

            It also allows one to swing the leg like a club, pounding a long bone against a rock outcrop or tree trunk until a spiral fracture develops (such fractures also date back to 2.5 million years ago).  Opening up a long bone allows access to the marrow, a highly desirable source of dietary fat, which comes out looking like a long pink sausage.  Fat is usually in short supply on the savanna, and a lot more of it is needed for building a bigger brain because the insulation wrapped about nerve cells is fatty.

            Stone toolmaking is first seen about 2.6 million years ago in Ethiopia, and the earliest of the bigger-brained Homo species have been traced back to 2.4 million years.   So first it’s toolmaking, then the spinoff.  While the dates may subsequently leapfrog one another as new evidence emerges, that is a typical ordering:  form follows function.  Behavior invents, using the klutzy old anatomy, then anatomy slowly changes to make the new behavior more efficient via the usual Darwinian process.

Consider the plight of the spinoff.  Life as a spinoff is not easy.   No, I have not shifted my origin story to the gas-log corporate campfire of the present day.  I am just beginning a little parable about the hazards of being a new species, isolated from the generous reservoir of culture and genes available from the “parent company.”

            A new species usually starts out as a small, isolated population.  Imagine, say, the big company’s branch office in Nairobi losing communication with the parent and having to manage on its own somehow.  No more imperatives to conform but also no more resources to draw upon, no buffer against hard times locally.  In biology, this is particularly precarious, as the usual swings in population with bad weather may take you down to zero – wipeout, with no hope of recovery.  The only thing that saves many small populations is reconnecting with the large parent population.  The infusion of immigrants will bring some of the gene variants that were lost simply by chance in the downsizings.

            But the immigrant infusion may also reverse “progress,”  as when Nairobi’s independent way of doing things is, upon communications being restored, re-conformed to the home office practice.  With all those forms to fill out again and auditors auditing the auditors, the innovative Nairobi office loses whatever edge it had developed during its isolation as an independent.

Brain size is carefully regulated in the course of growing up inside the womb.  Starting halfway through pregnancy,  the trajectory of human brain size rises above that of an ape.  The brain becomes relatively bigger, compared to the body.  So this is not recapitulation, where you first produce a monkey brain and then tack on some changes in the second act to keep enlarging the brain.  No, this is tinkering with things partway through the first act, early in gestation, much the way that the innovation of indoor plumbing eventually changed the way they built new houses, even in relatively early stages.

            Is this a new “gene for big brains” that has done the job?  That’s the usual way we tend to talk about it but, if we mean to be literal rather than metaphorical, it may well be wrong.  The regulation of the brain growth trajectory, if it is like other aspects of bodily development, is likely to be done by a committee of genes.  Some push, others pull, creating a channel that brain growth usually follows.   So instead of a new version of a gene, the big brain change might just be the elimination of one of the genes that held down the growth trajectory.

            As more of the chimp genome is sequenced and compared with the human genome, we will be looking for eliminations as well as additions.  Or, more likely, we will find the ascendancy of a less efficient gene variant.  Many genes come in various versions (called alleles), and that is how faster and slower, bigger and smaller, are usually varied in the body that the genes produce.

            For example, the receptors (“locks”) on the surface of a brain cell  – the ones that sniff out the neurotransmitter molecules (“keys”) released from upstream cells – can be made in several different ways.  The A1 allele makes a protein that is 30 percent less efficient than that made by the more common A2 allele of the D2 dopamine receptor.  This turns down the sensitivity of the downstream cell to dopamine.  This less sensitive A1 allele is found in a quarter of the population, and it may predispose its possessor to certain kinds of addictive behaviors (not just drugs but also gambling and obesity).  Perhaps it is merely a matter that these individuals do not satiate as easily, and their higher levels of ingestion thereby set off some chronic changes.

            The history of the field suggests that this preliminary explanation will be supplanted soon, but I intend it only as an example of how evolution works by having alternative versions.  Say, bluegrass and crabgrass.  The environment (how often you water and mow) tends to make some variants more common and others less so (which is what Darwin meant by his term “natural selection”).

            Let us now suppose that our isolated branch office, instead of dying out, is thriving at the time that lines of communication are restored.  Usually immigrants restore things and any idiosyncratic ways of doing business developed by the branch office in the meantime are conformed to the standards of the parent company.  But sometimes, the branch office people just don’t mix well with those from the parent organization.  The locals have become independent (a “new species”) and, rather than reintegrating, they go into competition with the “parent company.”  Size usually wins such competitions but occasionally the independent upstart will expand worldwide, maybe even forcing the parent company out of business.

            The economists would likely ascribe the success of the long-lost branch office to an “entrepreneurial culture.”  But in my parable, it was more a matter of losing a lot of the corporate culture, allowing a reformulation.  Gene versions get lost when things downsize, and sometimes that can make a big difference – such as setting brain growth in motion.

            Note that an archaeologist sampling the history every few years would likely get the story wrong.  What would be seen in most places would be the “sudden” replacement of the old-line company by the newbie.  It would look like the old company suddenly transformed itself into the newbie.  Only by sampling the history of the long-lost branch office would the gradual changes be seen, of both the isolation and the expansion of the new species.  And only a particularly fine sleuthing job would disclose when it was loss, rather than innovation, that played a key role in their improved corporate culture.  End of parable.

            With all of the droughts in Africa, hominid populations must have often been isolated from one another for considerable periods. Once they developed the skills to make a living in the grasslands, they would have taken advantage of the aftermath of a forest or brush fire, when new grass replaces the burnt woody plants for a few decades.  It was a good setup for getting stranded when the grasslands shrank in response to plant succession, all those woody plants starting to return and shrinking the size of the grazing herds.  There were many opportunities for a branch office to become independent – or go extinct.

No one really knows yet what toolmaking, a bigger brain, and more chaotic climate have to do with one another, but these relationships give one some food for thought.   Was the hominid of 2.5 million years ago still thinking pretty much like an ape, just with woodland overlays?

            They probably became much more daring, having to play games with those lions.  It became much more important to know what you could get away with.  They surely had an overlay of advanced hunting instincts by then, going considerably beyond the instinctive group maneuvers seen in chimps and bonobos when they hunt.  And their social instincts had likely changed as well, with more cooperation and sharing.