Popular Evolution Idea Challenged
A group of scientists at the Smithsonian Institution’s National Museum of Natural History has challenged one of the most widely held assumptions in human evolution: the notion that our early ancestors were prodded into existence in response to abrupt environmental changes during the Pliocene epoch.
Between 2.8 million and 2.5 million years ago, much of Africa became considerably colder and drier as glaciers began to creep across the Northern Hemisphere. During that same period, a new genus now named Homo emerged and split off from its shorter, less-brainy (but resolutely bipedal) predecessors, the australopithecines.
It seems improbable that this was simply a coincidence. Because environmental shifts frequently trigger development of new species, it has been broadly presumed that the African cool-down caused a sudden pulse of turnover in many animal species, including the advent of humans who began using stone tools.
But in today’s issue of the journal Science, the Smithsonian researchers report that they analyzed records of more than 10,000 specimens of 246 species from the famed Turkana Basin fossil sites in Ethiopia and Kenya and found no evidence of abnormally rapid evolutionary activity in the key period.
“It seemed like an important hypothesis to test,” said NMNH paleobiologist Anna K. Behrensmeyer, because “people were just assuming that human evolution was caused by this climate change.”
They had done so for more than a decade, thanks largely to the work of the preeminent proponent of the “turnover pulse” hypothesis, paleontologist Elisabeth Vrba of Yale University. Vrba argued convincingly that there had been a notably sharp spike in the number of new hoofed animal species in Africa around 2.5 million years ago. That proliferation, she demonstrated, took place at the same time that many heavy forests were giving way to savannahs or arid scrub lands, and temperatures were beginning to drop.
But some skeptics noted that the fossil record is very uneven. Owing to simple geological accident or other chance occurrences, scientists have obtained hundreds of specimens from some chronological periods (such as the era around 2.5 million years ago), and only dozens from others of the same duration. Periods with many fossils naturally provide evidence of far more extinctions and species appearances than do those with a sparse specimen inventory.
Consequently, it is hard to tell the difference between a genuine evolutionary surge and a merely copious roster of fossils. It is this potential “sampling bias” that the Smithsonian team set out to investigate.
The team constructed a large database of all the published fossil mammal discoveries from 425 sites near Lake Turkana on the Kenya-Ethiopia border, representing a time span from about 4.4 million years ago until the present. That region is a particularly prolific source of fossil evidence and has been studied so extensively that its various geological layers have been dated very accurately.
The NMNH researchers then analyzed the data to determine correlations between the number of new mammal species or extinctions (that is, “turnover”) reported for each 200,000-year period and the total number of specimens, or fossil abundance, for that period.
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They found that turnover remained remarkably constant between 3 million and 2 million years ago. No unusual “pulse” was evident from 2.8 million to 2.5 million years ago (the period of ostensibly critical climate change), and numerous forest-dwelling mammal species continued to thrive until 2 million years ago. The Smithsonian team did detect a substantial spike in turnover between 2 million and 1.8 million years ago but did not speculate on its cause.
In sum, the results suggest that “human evolution was much more a response to a prolonged series of climate fluctuations rather than any single shift,” said NMNH anthropologist Richard Potts. He suspects that one of early Homo’s distinctive advantages was the ability to succeed in a variety of settings. Perhaps “adaptation is not toward any particular habitat,” he said, but “to a tremendous variability of environments.”
The Turkana Basin discoveries make up the benchmark source used to date other African fauna finds, the Smithsonian team observes, and therefore is a highly appropriate locale for their analysis.
But it is possible, Behrensmeyer and Potts conceded, that different parts of Africa felt the climate change much more strongly than the water-rich environs of the Turkana Basin. “There may have been more stressful areas where species were being generated” faster, Behrensmeyer said, “but there were always these more buffered environments, too. I just don’t see any reason for [explosive turnover] to have happened so quickly.”
