Fossils heralded as the remains of 4- to 7-million-year-old hominids might actually come from apes.
Some anthropologists argue that Ardi and a couple other fossil creatures are really just apes, not human ancestors.
Human-like traits including small teeth and upright stance in apes may have mislead researchers.
The African primate known as Ardi and a couple of other fossil creatures widely regarded as early members of the human evolutionary family -- or hominids, for short -- may really be apes hiding in plain sight, two anthropologists say.
Hominid-like traits such as an upright stance and small canine teeth may have evolved independently in some previously excavated ancient apes, raising the possibility that alleged early hominids have been mislabeled, say Bernard Wood of George Washington University in Washington, D.C., and Terry Harrison of New York University.
Researchers have assigned African fossils dating to between 4 million and 7 million years ago to three groups of early hominids -- , Orrorin and Sahelanthropus -- and have suggested that these lineages evolved into later hominids. But any of the fossils used to build this argument could just as easily represent now-extinct apes or hominids from dead-end lines, the researchers conclude in the Feb. 17 Nature. Fossil finders have largely failed to acknowledge this classification conundrum, they assert.
Wood and Harrison's recommendation challenges excavators' standard practice of assigning a single evolutionary identity to new finds, based on comparisons with fossil and living creatures, without citing other possibilities.
The current debate in no way challenges the widely accepted notion that both the first hominids and ancestors of chimpanzees evolved from a common ape ancestor. But scientific opinions vary sharply on what that ancestor must have looked like.
"Researchers have to stop publishing papers that say, essentially, 'This fossil is an early hominid, so suck it up and accept it,'" Wood says. "Nature and Science could change this practice overnight if they wanted to."
Anthropologist Tim White of the University of California, Berkeley, responds that, at least for Ardi, comparative studies published in 2009 ruled out the possibility that she was an ape. White led the team that excavated and analyzed Ardi's 4.4-million-year-old partial skeleton.
Ardi's remains show many similarities to ensuing hominids in East Africa, White adds. He lumps all proposed early hominids into an Ardipithecus genus that evolved into the Ardipithecus genus by 4.1 million years ago. In contrast, Wood and Harrison suspect that early hominids -- whatever their identities -- branched out in many different evolutionary directions.
"With no new data, no new ideas, no new methods, no new hypothesis, no new experiments, no new fossils, not even a new classification, this paper will leave everybody wondering what's happened to the peer review process at Nature," White says.
Others welcome Wood and Harrison's warning. An upright stance and other features once considered hominid signatures evolved independently in many ancient primates, remarks anthropologist Tracy Kivell of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
Parallel evolution of these traits makes the evolutionary status of proposed early hominids "more uncertain than originally described," Kivell says.
Scientists currently have no good way, either with bones or genes, to test the hypothesis that proposed early hominids are ancestors as advertised, remarks anthropologist John Hawks of the University of Wisconsin–Madison.
Wood and Harrison discuss two cautionary tales of hominid classification gone awry. In one case from more than 30 years ago, scientists thought that 12-million-year-old Asian fossils from a creature called Ramapithecus belonged to a hominid until further finds pegged it as an orangutan ancestor.
A second case concerns 7- to 8-million-year-old Oreopithecus, designated a hominid about 50 years ago. This creature walked upright with a shuffling gait and displayed some other hominid-like features. But by the 1990s Oreopithecus had been unmasked as an ape that evolved unusual traits on several Mediterranean islands.