Walking With Primates
This week news services were all a-twitter about a 47-million-year-old fossil primate from the famous Messel deposits of Germany. Named Darwinius masillae and described in the journal PLoS One, the lemur-like primate was heralded as being a transitional form between a group of extinct primates called adapids and anthropoid primates (monkeys and apes). As it turns out the fossil may not be all it has been cracked up to be, but it is still a spectacular find that represents one branch of the primate radiation that occurred after the mass extinction that killed off the dinosaurs at the end of the Cretaceous. Creatures like Tyrannosaurus perished, but primates survived.
Tracing the record of the earliest primates is a challenge. Since primates started off small and lived in forested habitats their fossils are extremely rare, and most fossils that are found are teeth. This can make comparisons between these creatures difficult, and the relationships among early primates or primate-like creatures are controversial. The fact that some molecular studies places the origin of primates even further back in the Cretaceous, about 85 million years ago, makes things even more complicated as no verifiable primate fossils have yet been found from that age. Despite these complexities, however, scientists do have a broad outline of early primate evolution.
One of the earliest primate-like creatures was Purgatorius, a tree-shrew-like mammal that lived right around the end of the Cretaceous 65 million years ago. Whether it was one of the first primates or only closely related to the first primates is still controversial, but it does seem to represent what the ancestors of primates were like during the time that dinosaurs were the dominant land-dwelling vertebrates.
After the mass extinction, mammalian evolution exploded. Mammals were no longer under the feet of dinosaurs, and among the groups that diversified were primate-like creatures called plesiadapiformes. Whether these creatures were true primates or just very primate-like is still being debated, but they underwent a boom and bust during the Paleocene (about 65 to 55 million years ago). In many ways these creatures were somewhat squirrel-like, with clawed hands and eyes on the sides of their heads, but at the very least they seem to be the closest extinct relatives to other primates.
The creatures that are regarded as "true" primates flourished during the Eocene (about 55 to 33 million years ago), and can largely be placed into two groups: the adapids and omomyids. The adapids were lemur-like primates, while the omomyids closely resembled living tarsiers, but both had forward-oriented eyes and adaptations to life in the trees. Both these groups are relevant to yesterday's big announcement.
According to the new paper, Darwinius is an adapid, and many scientists presently regard this group as being more closely related to modern lemurs and lorises than to monkeys or apes. Many paleontologists who study extinct primates favor omomyids and ancient tarsiers as being closer to monkeys and apes, but the authors of the new paper don't think so. In the paper itself they claim that Darwinius belongs to the same large group of primates, haplorrhines, as tarsiers, monkeys, and apes, thus placing adapids in a position to potentially become our ancestors. This conclusion has caused the scientists involved in the study and the popular media to herald it as a "missing link" that connects us to other primates.
Unfortunately, however, the scientists who wrote the paper did not conduct a detailed evolutionary analysis of the new fossil or its relationships to other primates. The fossil is spectacular, the first fossil primate to be find in such a state of exceptional preservation, but it has been oversold by the History Channel (who organized the media hype) and the scientists involved in the study. They simply did not do the work to support the conclusions they drew from the fossil, and the real relationship of Darwinius to other primates will have to wait for further studies.