Human evolution research is not for the faint-hearted. Hominin fossils are rare and hard to find. And more often than not, no sooner do anthropologists announce a big discovery than other researchers argue that they have it wrong. The next chapter in such a scenario unfolded last week, when scientists attending a meeting*at the Royal Society in London resurrected a debate about a single, crucial hominid specimen: a 3.5-million-year-old cranium named Kenyanthropus platyops—“the flat-faced man of Kenya” (shown at right).
Discovered in 1998 by a team including paleontologists Meave Leakey of the National Museums of Kenya in Nairobi and Fred Spoor of University College London, K. playtops suggests a greater degree of diversity in the human family tree than previously suspected: two species of hominids, not one, in the crucial period between about 4 million and 3 million years ago. That’s the time of Lucy, Australopithecus afarensis, whose lineage is thought by many to have given rise to our own genus, Homo.
But there was one nagging problem: The Kenyanthropus cranium, discovered west of Kenya’s Lake Turkana, was cracked and distorted, making it possible that some features that set it apart from A. afarensis—including its flat face and tall, vertically oriented cheek bones—could be artifacts. Paleoanthropologist Tim White of the University of California, Berkeley, argued in a 2003 Perspectives in Science that K. platyops probably fell within the range of variation among known A. afarensis fossils and might simply represent an “early Kenyan variant” of that species.
In London, Spoor responded to such arguments with a new, detailed study. He concluded from computed tomography scans that the upper jaw, or maxilla, had suffered much less distortion than the rest of the cranium. So he focused on that bone, correcting for the distortion present. He measured distances among several “landmarks” on the maxilla, including the point at which the cheekbone attaches to it, the extent of its forward projection, and the orientation of its tooth sockets. He used the same landmarks on A. afarensis specimens, the roughly 4-million-year-old A. anamensis, later australopithecines, and modern humans, chimps, and gorillas. Then he crunched the measurements in a computer-assisted analysis called principal component analysis to reveal the variability among the specimens. The result: Kenyanthropus fell cleanly outside the range of variation in all the other samples. “Species diversity existed at 3.5 million years ago, and this justifies assigning a new taxon,” Spoor concluded.
But White, who was present and has long argued that there is no evidence for more than one lineage of hominids at this time, wasn’t convinced. When the talk was thrown open for discussion, White took the microphone and began firing questions at Spoor about the degree of variation of the cheekbone position among specimens of A. afarensisand other hominin species. “We took that into account,” Spoor responded, “and I just showed you a graph” about it. “I didn’t ask you whether you took it into account; I asked you what it was,” White said. Spoor, clearly frustrated, told the audience that he had no vested interest in this debate. At that point, the session chair interrupted and invited everyone to break for coffee, but Spoor and White continued to debate between themselves for the next half-hour.
Spoor’s study, like the others presented at the meeting, will be published in the Philosophical Transactions of the Royal Society B: Biological Sciences in Spring 2010. When it is, the debate will no doubt continue.
*The First 4 Million Years of Human Evolution, London, 19-20 October 2009.