Mystery hominin turns into mystery hominoid

Fossil teeth can be tricky things. In 1922 paleontologist H.F. Osborn believed that he had found the first evidence of an extinct fossil ape from North America on the basis of a worn molar from Nebraska, but it later turned out to be the tooth of a prehistoric peccary. Four years later, by contrast, Davidson Black named a new species of ancient human on the basis of a handful of teeth recovered from Dragon Bone Hill in China. These turned out to belong to Homo erectus. Both paleontologists made bold steps on the basis of sparingly little evidence, but with entirely different outcomes.

Last week paleoanthropologist Russel Ciochon revealed that lately he has been feeling a bit more like Osborn than Black. Back in November of 1995 Ciochon was one of the lead authors on a Nature paper entitled "Early Homo and associated artefacts from Asia" which proposed that Homo habilis-like hominins occupied Asia around 1.8 million years ago. This was based upon a jaw fragment containing a premolar and a molar found in association with proposed stone tools and other fossils in Longgupo cave in Sichuan province, China. If this interpretation was correct, the jaw fragment would represent an early radiation of Homo out of Africa and (some claimed) would throw support to the multi-regional hypothesis of human origins.

Subjectivity does play a role in these sorts of interpretations, however. As Sigrid Schmalzer demonstrated in The People's Peking Man hominin fossils from China, and Homo erectus fossils specifically, have often been thought of as ancestral to people living in that region. National pride has influenced the interpretation of the fossils such that certain cultures in Asia had specific origins not shared by other groups. This is a simplification, and I in no way intend to assert that all Chinese paleoanthropologists assert this view, but it is a trend worthy of comment that fits into the traditional "Your stuff is shit, but my shit is stuff" dichotomy that pervades modern paleoanthropology.

Regardless of how the conclusions about the jaw fragment were formed, however, Ciochon has now stepped forward to express his doubts about his previous interpretation. The fragment does not represent an early hominin, he says, but an as-yet-unknown species of ape that lived in the forests of what is now China during the early Pleistocene. Where Ciochon had previously compared the teeth to those of the large extinct ape Gigantopithecus and the hominin Homo erectus, the Longgupo cave teeth more closely resembled those of the more ancient (~6-7 mya) ape Lufengpithecusis and other hard-to-identify specimens from surrounding deposits of Early Pleistocene age. Given these resemblances it appears more likely that the "pre-erectus" hominin that Ciochon thought the teeth represented was truly some as-yet-unknown species of ape.

Ciochon promises that he and other scientists are already working on trying to identify the "mystery ape" from Longgupo, but this is surely not the last time that we will have to revise hypotheses of hominoid evolution based upon fossil teeth and little else. In 2007 a team of Ethiopian and Japanese paleoanthropologists described the ~10 mya ape Chororapithecus on the basis of a handful of teeth in the journal Nature. They proposed that Chororapithecus was an early gorilla that pushed back the date of the last common ancestor of gorillas and chimpanzees + humans, but with so little fossil material available it is difficult to tell whether 1) Chororapithecus truly is an early gorilla, and 2) whether it is at all informative about evolutionary divergence dates for hominids. Many more fossils are needed to better ascertain its position, especially since we have almost no knowledge of the fossil record of gorillas and chimpanzees.

Not long after the announcement of Chororapithechus another African ape from ~9.8 mya deposits in Kenya named Nakalipithecus was described in PNAS on the basis of teeth and jaw fragments. While it was heralded as another piece of evidence that the ancestors of gorillas, chimpanzees, and humans evolved in Africa (and not Eurasia) the dearth of fossil material made it difficult to determine the relationship of Nakalipithecus to other apes, both living and extinct.

Clearly Chororapithecus and Nakalipithecus were fossil apes that lived in Africa about 10 million years ago, but their evolutionary relationships to other apes (and thus their significance to the timing of our evolution) is still difficult to determine. Not only do we require more complete specimens of these genera, but discoveries of new species and genera will be increasingly important to filling in the picture of hominoid evolution. The ape fossil record is generally very fragmentary, and with the pressure to make discoveries sound unique, important, or sexy, sometimes interpretations that hinge on future discoveries can be made prematurely. Fortunately, though, discoveries and hypotheses previously made will be continually tested and held up for scrutiny. This process is ongoing, from one generation of scientists to the next, and I look forward to the day when we have a better idea of how Chororapithecus, Nakalipithecus, and the Asian "mystery ape" fit into hominoid evolution.