IT鈥橲 been called the cradle of humanity, but the significance of East Africa鈥檚 Turkana basin in human history is still unclear. Now some ancient herbivore teeth are revealing the region鈥檚 special climate around the time our genus Homo first appeared.
Turkana, part of the Great Rift Valley straddling Kenya and Ethiopia, has been a hallowed site for the study of human evolution ever since Maeve and Richard Leakey began uncovering fossils there in the 1960s. Ranging from Australopithecus to our own species, Homo sapiens, the most striking finds include Turkana boy, a Homo erectus fossil that is the most complete early human skeleton ever found.
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But was Turkana a recurring site for major events in human evolution, perhaps because it was a humid refuge for our ancestors during particularly dry periods, or simply a good environment for preserving fossils?
Now the teeth of herbivores are helping to answer this question. By studying fossil teeth from across the region, at the University of Helsinki, Finland, and his team have pieced together a record of the region鈥檚 temperature and rainfall that goes back 8 million years 鈥� well before early humans first appeared.
Their data show that East Africa as a whole became drier between 3 million and 2 million years ago 鈥� the period when our genus Homo first emerged. But the Turkana basin began to dry out earlier.
Fortelius thinks the basin鈥檚 early shift meant it could have acted as a 鈥渟pecies factory鈥�. Since it was ahead of the trend, new species that evolved there were adapted for the drier environment that later became widespread.
鈥淪tatistically, you would expect many species that evolve in an area that鈥檚 ahead of its time to be more successful, on average, than an area that鈥檚 lagging behind,鈥� he says. 鈥淚t might mean that, in East Africa at least, the Turkana basin might have contributed a disproportionate amount of new biodiversity for the region.鈥�
Fortelius presented his research at a meeting to celebrate the 70th birthday of Richard Leakey, held at the Royal Society in London last month. 鈥淚 personally think it鈥檚 a great idea,鈥� says at Columbia University in New York. 鈥淔or the first time he鈥檚 showing how unique the climate is in specific parts of Africa.鈥�
De Menocal has been using ocean sediments to reconstruct the history of Africa鈥檚 climate. Fossil plankton layers correspond to more humid conditions, while silty grains of dirt indicate more arid grasslands, giving an idea of the average climate across a broad region of north-east Africa.
But Fortelius鈥檚 use of teeth is far more specific, says de Menocal, allowing different areas to be compared. 鈥淚f you want to understand how climate has changed where the fossils are actually found, that鈥檚 the kind of approach you need,鈥� he says.
Fortelius looks at the cutting edges and height of herbivore teeth that were present at a certain time. Short teeth, like those of elk, suggest a humid environment with lush vegetation, while tall teeth, like those of horses, are suited to wear and tear from chewing grit in a grass-based diet, indicating a dry climate.
鈥淭he evidence comes straight from the horse鈥檚 mouth,鈥� says Fortelius. His team has previously used this approach to estimate recent global rainfall patterns, with the results .
The evidence from the teeth supports the story that the broader region began to dry up 3 million years ago, as suggested by de Menocal鈥檚 sea sediment work, and analysis of the isotope carbon-13, which is more abundant when plants adapted to arid conditions, like grasses, have been present.
鈥淭he drying is coincident with a lot of major events in human evolution,鈥� says de Menocal. It coincides with the appearance in the fossil record of the first members of the Homo genus, along with Paranthropus, a group of hominins known for their robust skeletons and grinding teeth. Australopithecus, an older group, disappeared around the same time.
鈥淭he drying out of East Africa coincides with the first members of the Homo genus in the fossil record鈥�
The specific role of the climate shift in these events is unclear, but it would have changed what foods were available. show that Paranthropus鈥榮 diet was mostly derived from grasses, while the doomed Australopithecus almost exclusively ate plants that weren鈥檛 so well adapted to hot temperatures. Early Homo species seem to have eaten a mixture of grasses and non-grasses.
Other incidences of human speciation, extinctions and migrations seem to overlap with periods of unstable climate, and some researchers have suggested that traits such as large brains and bipedality evolved to help our ancestors cope with changing climate. But inferring clear relationships between climate change and evolutionary events is fraught with difficulty, says at the French National Centre for Scientific Research.
鈥淭he problem is that when you look in detail at what we have in the field, it鈥檚 not so obvious,鈥� says Roche. It had been thought that the first stone tools were linked to a drying climate, but this year, that are 700,000 years older than any found previously. 鈥淣ow we have to rethink everything,鈥� she says.
at George Washington University warns against making assumptions about where evolutionary events happened from the location of fossils. 鈥淭he problem is, the drunk looks for his keys under the lamp post because that鈥檚 the only place he can see,鈥� he says. 鈥淭here鈥檚 an awful lot of Africa for which one doesn鈥檛 have the same evidence for the same time.鈥�
(Image: Greatstock Photographic Library/Alamy Stock Photo)
Indonesian cave of bones
Africa isn鈥檛 the only continent that鈥檚 thrown up intriguing human fossils. In 2005, researchers discovered remains of the diminutive Homo floresiensis, nicknamed 鈥渢he hobbit鈥�, in a cave on Flores Island, Indonesia.
The bones suggest a human only 1 metre tall, living as recently as 18,000 years ago 鈥� long after other early human species, including the Neanderthals, had disappeared.
But a decade later, controversy still remains over whether the hobbits were a separate species, or simply a population of unusually small Homo sapiens.
The only H. floresiensis skull found so far housed a brain as small as a chimpanzee鈥檚, leading sceptical researchers to contend that it belonged to a malformed H. sapiens who had a disease impairing neural development.
To lay the debate to rest, more bones are needed, but so far researchers have drawn a blank. This could be because they have been looking in the wrong place.
But that could be about to change. In 2006, Michael Gagan at the Australian National University in Canberra and his team discovered that the hobbit cave had a hidden basement chamber. Inside they found some more recent animal bones and stone tools during their initial excavations.
He hopes that deeper, older layers will contain H. floresiensis bones. 鈥淲ho knows what amazing ancient bones could be buried there?鈥� says Gagan. Colin Barras
This article appeared in print under the headline 鈥淗ot climate fired-up human evolution鈥�
