Seperti halnya pemahaman evolusi tektonik di Indonesia, pemahaman tentang jalannya evolusi manusia juga menjadi lebih rumit dan makin diperdebatkan setiap kali timbul penemuan baru. Jadi, sebenarnya kapan asal-usul keluarga manusia itu, kapan sebuah fosil layak disebut memang ini hominid � leluhur manusia, dan bukan dari primata golongan leluhur kera. Fosil-fosil �hominid� yang ditemukan lebih tua dari 4.5 atau 5 juta tahun (lebih tua dari Pliosen) riskan sekali untuk jadi bahan perdebatan sengit.
Setiap paleoantropologists tentu tak mau mengulang �kasus Ramapithecus� yang memalukan itu. Tahun 1932 sebuah fosil rahang atas ditemukan Edward Lewis dari Yale University dan rekonstruksi2 berikutnya terutama oleh Simon dan Pilbeam dengan penemuan2 baru sampai awal tahun 1960an menghasilkan sebuah makhluk kecil mirip kera dan dianggap spesies hominid tertua, inilah Ramapithecus, yang umurnya 15 juta tahun yl. Maka diyakini bahwa manusia mulai berevolusi sejak 15 juta tahun yl. Akhir tahun 1960an teori ini goncang oleh dua orang ahli biologi molekuler/biokimia Wilson dan Sarich dari University of California di Berkeley yang menyatakan bahwa hominid mulai berevolusi terpisah dari golongan kera sejak 5 juta tyl, jadi Ramapithecus adalah leluhur kera bukan manusia. Penelitian Wilson dan Sarich didasarkan pada analisis kimia darah manusia hidup dan kera Afrika. Mereka membandingkan perbedaan struktur darah manusia dan kera. Perbedaan itu bertambah dengan kecepatan tertentu akibat mutasi. Semakin beda maka semakin banyak mutasi telah terjadi. Kecepatan mutasi dihitung dan dilacak hitung mundur sampai kapan mutasi pertama terjadi. di situlah leluhur kera dan leluhur manusia berpisah dan itu adalah 5 juta tyl. Perdebatan sangat sengit terjadi antara para paleoantropologists dan para ahli biokimia. Setelah lebih 10 tahun, pada awal 1980an, kemenangan secara tidak langsung ada di kubu para biochemists sebab para paleoanthropologists menemukan fosil Ramapithecus yang jauh lebih lengkap, dan mereka mengakui bahwa Ramapithecus bukan hominid, tetapi leluhur kera, maka 15 juta tyl mereka coret sebagai asal hominid, mungkin benar di sekitar 5 juta tyl seperti yang dibilang para ahli biokimia. Tahun 1994 Aridpithecus ramidus ditemukan di Ethiopia dan umurnya disebut 4,5 juta tyl, ini sudah diakui sebagai hominid tertua yang menurunkan golongan Autralopithecus. Sekarang subspesiesnya yang lebih tua ditemukan, Aridpithecus ramidus kadabba yang berumur 6 juta, juga spesies lain yang seumur dengannya (Sahelanthropus tchadensis dan Orrorin tugenensis). Jangan2, A. kadabba, Sahelanthropus dan Orrorin akan mengulang kasus Ramapithecus ?! Hanya dari geligi atau rahang merekontruksi spesies baru dan menempatkannya di titik ekstrim bisa membahayakan. Kasus Ramapithecus sebenarnya bisa mengajarkan bahwa adalah bahaya menarik kesimpulan berupa kekerebatan evolusioner dari kemiripan ciri-ciri anatomis. Dulu Ramapithecus diberi gaya hidup lengkap berdasarkan hanya bentuk gigi taring, jika terdapat satu ciri hominid maka segala ciri lain semacamnya diandaikan ada juga (hal ini sekarang diulangi untuk A. kadabba). Hati-hati, para geologist pun sering tergiring seperti itu. Salam, Awang Rovicky Dwi Putrohari <[EMAIL PROTECTED]> wrote:March 05, 2004 (Scientific American) Fossil Human Teeth Fan Diversity Debate The discovery in Ethiopia's Middle Awash region of a handful of nearly six-million-year-old teeth is adding fuel to a longstanding debate among scholars of human evolution. At issue is whether the base of our family tree is as streamlined as a saguaro, or as shaggy as a shrub. When it comes to classifying fossils, paleoanthropologists generally fall into two camps. There are the splitters, who parse the human fossil record into numerous genera and species; and the lumpers, who recognize fewer, more variable taxa. Both factions agree that several hominid species co-existed during the later stages of human evolution, between three million and 1.5 million years ago. The number of forms that shared the landscape shortly after humans diverged from chimpanzees, on the other hand, is vigorously disputed. In a report detailing the new findings, published today in the journal Science, Yohannes Haile-Selassie of the Cleveland Museum of Natural History and his colleagues assign the ancient teeth to a new hominid species, Ardipithecus kadabba. In light of the discovery, the team argues, remains previously attributed to the subspecies A. ramidus kadabba should now be considered part of the new species, which is older and more primitive than A. ramidus. Particularly important in their analysis are the upper canine and lower third premolar that turned up. All fossil and modern apes, particularly males, have large, tusklike canines that are continually honed against the lower third premolars, which keeps them sharp for fighting (mostly over access to mates). Humans, in contrast, have smaller, more incisorlike canines, which scientists have interpreted as indicative of increased male cooperation. For their part, the A. kadabba canine and premolar exhibit a mix of apelike and hominidlike traits, prompting Selassie to speculate that this species might be the first on the human line after the chimp-human split. A. kadabba is not the lone contender for the title of earliest member of the human lineage. Two other putative hominids dating to the late Miocene epoch--Sahelanthropus tchadensis from Chad and Orrorin tugenensis from Kenya--surfaced in 2002 and 2000, respectively. But Selassie and his collaborators suggest that the teeth of these specimens indicate that they are very similar to A. kadabba. On the basis of the available evidence, they contend, all three may belong to the same genus, or even species. A contrary view comes from David R. Begun of the University of Toronto, who counters that the A. kadabba, Sahelanthropus and Orrorin dentitions differ in important ways. "Rather than a single lineage, the late Miocene [hominid] fossil record may sample an adaptive radiation, from a source either in Eurasia or yet undiscovered in Africa, the first of several radiations during the course of human evolution," he writes in an accompanying commentary. But the level of uncertainty about the fragmentary fossils known thus far makes it impossible to reconcile these differences of opinion between lumpers and splitters. "The solution is in the mantra of all paleontologists," he concludes. "We need more fossils!" --Kate Wong _________________________________________________________________ Add photos to your e-mail with MSN 8. 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