East Africa's Rift Valley is familiar to many travelers who find a series of lakes beneath the rift escarpment. Over the ages many such lakes have come and gone, forcing the residents of the area to make do with things as they were during their short lifetimes.
At last I’ve been shown an evolutionary reason for developing the ability to say an infinite variety of things, thanks to the current (November) special issue of the Journal of Human Evolution devoted to the African climate where the human line of descent emerged. (Table of contents here) It is a great issue, likely to have a revolutionary impact on our understanding of the conditions that gave rise to Homo. In particular, adaptation to savanna grasslands appears to have been a much later pressure than we have supposed.
In one of the early posts on this blog I discussed the possibility that humans evolved along lake shores instead of out on the open savanna (see post). Now that idea is looking even better.
Visitors to East Africa find a very mixed terrain of desert, wetland, grassland, and forest spread across mountain ranges, vast flat spaces, the complicated Rift Valley faults, and numerous volcanic peaks. Ten million years ago, however, it was a relatively flat terrain covered with rainforest. Humans evolved in this land while the terrain itself was undergoing a series of great transformations; climate-wise it was the best of times, it was the worst of times.
It was the best of times when the Rift Valley was very wet with large, permanent bodies of fresh water available. (You understand, of course, that ‘permanent’ in this geological context doesn’t mean there forever. The water was available throughout the year and for many years, so that from the point of view of local residents the water was an enduring, reliable feature of the landscape.)
Those same wet periods were also the worst of times because outside of the Rift Valley the terrain was very dry. The landscape there sounds to me like what the East African colonials used to call ‘a million miles of bloody Africa,” large, arid areas given over to brush that can support very few animals. The animals of any size that do survive, manage it by keeping on the move, following whatever water they can. Moving out during the rainy season and, when the rains stop, contracting into wherever permanent water exists. Humans in these areas tend to be nomads, e.g., the Masai people in southern Kenya and northern Tanzania, and the Turkana people of northern Kenya.
A farmer on the edge of Masai land once summed up for me what it was like to be settled in such unsteady country: “When it does not rain,” he told me, “we are hungry; when it does rain, we eat.” Behind his thatched hut was a dry creek bed that probably remained dry eight or nine months of the year.
It turns out that much of the speciation of our ancestral lines took place during these best/worst times. According to the issue’s most striking sentence, even though these contrary periods comprised less than a third of the time during the period from 5 million years ago to half a million years ago “12 of the 15 hominin species first appeared in one of these extreme ‘wet-dry’ periods" [p. 459]. If chance alone were at work, we would expect only 5 of the species to first appear during such a period. (Granting, of course, that counting species is subject to much dispute.)
At the end of this post I’m attaching a summary of the relation between climate and appearance of hominin species. The picture I get from putting this appendix together is that the early periods of wet rift areas and dry outlands were a time of speciation. For example, three hominin species appeared during the wet-dry event of 3.5 million years ago. Two of those species appear to have gone extinct when the wet-dry climate disappeared, while the surviving species (Australopithecus afarensis) lasted through the next wet-dry event of 3 million years ago. During that later event Australopithecus africanus appeared and endured while afarensis went extinct once the wet-dry times ended. There is something of a marathon race quality to this. Afarensis made the first lap, then handed the baton to africanus, who ran to the next wet-dry event where it passed the baton to the first Homo species who started running while africanus disappeared.
This story seems to make sense. The wet-dry events produce some local lush periods when species can prosper in their separate Edens. Then the lakes dry up and everybody is forced to compete over the broad, dry lands. Only one species survives this rough competition and it pushes on through the next lush period, only to face losing competition to the new, improved competitor who emerged during the later wet-dry.
I would like to know if a similar story holds outside the human line of descent. How were elephants, for example, doing during these eons? Were they also speciating like mad during the wet-drys and then dwindling down to a single species during the long, dry spell that followed? And were they too producing a new winning species during each of the wet-dries or was there a basic model elephant species that won every time the competition got rough again? We tend to see the move from afarensis to africanus to habilis as a steady progress, but normal biology replacements merely reflect better adaptations to new conditions. Were elephants too being pushed in some direction that made them smarter and more flexible than the elephants of five million years ago? How about lions? A little comparative information would help here.
The human story changes with the appearance of Homo erectus and ergaster during a wet-dry that lasted from 1.9 to 1.7 million years ago. Ergaster did go extinct shortly after the next (and final) wet-dry event of 1 million to 900 thousand years ago, but no new species appeared and erectus emerged from the other side without new competition. The later species of Homo—heidelbergensis, neanderthal, and sapiens—emerged without the wet-dry push.
Does any of this help us understand the emergence of language or culture in general? If you look at the details of language, the information seems way too general to support any conclusions, but if you look at Noam Chomsky’s favorite fact about language—that it carries the potential for making an infinite number of different statements—this climate/species information seems directly on target.
Species that adapted too specifically to the wet conditions around the lakes would have gone extinct when the long droughts followed. Looking at a wonderful chart [p. 482] provided in the journal we see exactly that. Hominin species appearing when a wet-dry begins and disappearing when the same event finally ends. One might expect a new species adapted specifically to the drought conditions to emerge, but that doesn’t happen. Instead, a species that prospered during the wet-dry event chugs along through the drought as well, suggesting that it was flexible rather than adapted to precise conditions.
We tend to assume that it’s the strongest and most aggressive species that survives, but that may not be true in this story. The strongest and most aggressive species may have been able to hold and adapt to the wet lakeshore, while the weaker and less aggressive one had to move between lakeshore and the drier lands away from the water. So when the wet times finally disappeared, they got the last laugh while the old lords of the lakeshore were at a loss in how to respond.
In this scenario we have a kind of pump producing ever more adaptable species. A wet-dry event ends with a species adaptable enough to move between lakeshore and brush country. That species makes it to the next wet-dry event, when speciation processes produce an even more adaptable species that emerges triumphant during the next long drought. This pump keeps producing more adaptable species until it finally gives us Homo erectus, a species so adaptable that it moved far beyond the African brush country into Asia and possibly even parts of Europe. We tend to be a bit snooty about erectus, what with its million-year taste for the same axe, but it spread far and wide into all kinds of ecosystems. That capacity to fill niches should count for something. And how was it adapting to all those different worlds? I’m guessing it found many new things to say and ways to say them.
Ardipithecus ramidus: This is a possibly bipedal, possibly forest-dwelling ape from modern-day Ethiopia. It was originally dated at 4.4 million years, which sets it in one of the wet-dry periods. Feeders to alkaline lakes appear to have provided reliable water in Ethiopia, although soda lakes themselves are not a good source of drinkable water. The species has more recently been given a much earlier date of 5.8 million years, well before the first of the wet-dries; on the other hand, from a chart provided [p 479] it looks as though some small lakes appeared in Ethiopia at about that time, so water was available.
Australopithecus afarensis: the species famous for producing the Lucy fossil. A chart in the journal [p. 484] puts its origin at about 3.6 million years ago, during a wet-dry period and extends to about 2.8 million years ago, just after a wet-dry period, when Australopithecus africanus first appeared. The Lucy fossil itself is dated to 3.1 million years ago, and checking the journal’s wonderful charts, I find that right then Lucy’s region of Ethiopia was enjoying deep, freshwater lakes, suggesting a reason for finding a good fossil from that time. It was a lush period when perhaps there were more such creatures around, thereby increasing the odds of one of them fossilizing and surviving to be found.
Two other species appeared during this wet-dry phase along with afarensis. They were Kenyanthropus playtops and Australopithecus bahrelghazali, both species who seem to have been born and gone into extinction during the same wet-dry phase that gave birth to afarensis.
Homo habilis/Homo rudolfensis both appear late in the wet-dry period that lasted from 2.7 to 2.5 million years ago. This was also the wet-dry period that saw A. africanus go extinct. Habilis has been found down in the Olduvai area which appears to have been dry during this wet-dry period. Indeed, right next to Olduvai, Mount Ngorongoro was undergoing persistent volcanism, making for periodically stressful living conditions. Lake Rudolf (these days known as Turkana), where the other species is found, was also dry. It would be most convenient for the proposal linking speciation with wet-dry periods if a parent Homo could be located in the central Kenyan rift area, say around modern Lake Baringo, which did see large, fresh water lakes at this period.
We have to be careful when we talk about a lake period lasting 200 thousand years. The Baringo lakes actually came and went 5 times during these years, with interruptions that lasted 23 thousand years. Those 23,000 years are a geological blink, but very long for the generations that lived through them.
Homo ergaster/Homo erectus arose, according to the charts, during the next lake period (1.9 to 1.7 million years ago) and habilis disappeared shortly after that period. This period saw large lakes in Olduvai, central Kenya, Turkana, and Ethiopia. This was the period when the El Niño system first appeared in the Pacific, and although that seems far from East Africa it has impacts there. To this day, El Niño in the Pacific means drought in East Africa, so there was plenty of year-to-year variability in rains, meaning that the areas away from the permanent freshwater lakes was quite unreliable.
The next, and final, of the wet-dry periods came a million years ago and with it the end of Homo ergaster. But no new species of Homo arose then and the Homo species that appeared later—antecessor, heidelbergensis, neanderthal, and sapiens—arose in different circumstances. The main reasons for the change was probably that the rifting process had settled down. Today’s basic escarpment structure was settled by 1.2 million years ago and the northern hemisphere’s deep ice-age cycle dominated much of the earth’s climate. East Africa was far from the glaciers of North America and Europe, but the ice-ages still affected the region by drying it out. Humidity was down, rain was down as much of the earth’s water was locked in ice.