Tiny Ancient ‘Hybrid’ Primate Revealed to Be ‘Crucial Link’ In Anthropoid Evolution

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Fifty five million years ago, a tiny, primate species weighing less than one ounce (28.35 grams) lived, ate, mated and died amongst the lush vegetation and tree-ringed lakes of a tropical forest that once thrived in central China.

We know of this creature — officially named Archicebus achilles — thanks to a well-preserved (nearly complete) fossil found in the hardened sediment of what was once an ancient lake bed. The fossil was found nearly ten years ago but it has taken all that time for scientists to analyze its unique features and determine it proper place in the anthropoid family tree. The findings will help paleontologists answer key evolutionary questions — like how, when and where our earliest primate ancestors evolved.

The unique fossil find is also relatively rare; its hind legs and almost all its tail vertebrae are very well-preserved. This allowed scientists to develop a finer picture of the creatures lower half. Plus, they were able to reconstruct key features of its partially crushed skull using high-energy x-rays. This reconstruction then allowed the investigating team to compare anatomical features with other known (living and extinct) primate species.

The Results and Details of the Analysis

Following the decade-long analysis, the research team subsequently determined that A. achilles is most likely an early ancestor of modern tarsiers. Tarsiers are small, nocturnal (and currently endangered) primates that are found only on a small number of islands in Southeast Asia. Their nocturnal lifestyles are aided by their unusually large eyes (the better to see in the dark, of course), giving them a sort of “alien” appearance. Its long heel bones permit powerful leaps to capture small prey or perhaps avoid predators.

Tarsiers represent a split in the evolutionary branching of the primate family tree; with the other branch leading to ancestors of modern marmosets. As with many early primate fossil finds (e.g., “Ida”; Darwinius masillae), much debate centers on whether a given fossil comes via the tarsier or marmoset branches. The current view is that modern primates descended from the marmoset branch.

“A. achilles sits at that critical part of the tree right where the tarsier branch is splitting away from the anthropoid branch,” explains K. Christopher Beard, a vertebrate paleontologist at the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, and a co-author of the study

Despite a few obvious physiological differences, the team is convinced that these strange creatures are indeed part of our anthropoid family tree — the phylogenetic grouping that includes all known primates (monkeys, apes and humans). This long and conclusive analysis of a single fossil specimen helps scientists answer some long-standing question about the appearance and behavior of out most ancient ancestors — such as it size, its eating habits, its preferred habitat and when it was most active (night or day, etc.).

And despite is evolutionary placement next to ancestors of tarsiers, the diminutive creature has features that are distinctly anthropoid (i.e., more “monkey like”) — most especially its feet which have short toes and a short heel bone. These characteristics are quite different from modern tarsier feet and very similar to the feet of South American monkeys, such as marmosets* (which have some more “primitive” features, like claws, compared to other monkeys, which have nails).

And unlike the tarsier, A achilles had relatively small eye sockets — meaning that its eyes had not yet evolved into the larger orbs common to tarsiers and which allows them to be nocturnal. Thus, the scientists concluded that the creature was most likely active during the day (which might partly explain why the species went extinct).

But, neither is the creature completely marmoset-like; it seems to be a type of “hybrid” species. according to Beard:

“What this new fossil is telling us is that the common ancestor of tarsiers and anthropoids really was a hybrid. It would not have been in any way completely monkey-like, but it certainly wasn’t completely tarsier-like, either. It had certain features of both lineages already present.”

Overall, the team’s analysis strongly supports the hypothesis that our earliest primate ancestors were diminutive, “shrew-like creatures” that survived on a calorie-rich diet of mostly insects.

What’s more, the location of the unique fossil find (China) also adds more support to a once-controversial hypothesis: that the earliest primates came from Asia, not Africa. This theory was first proposed by Beard and a few others in the 1990′s, and was roundly rejected by the majority of scientist in their discipline at the time. however, since that time, there has been a fairly constant record of fossil primate finds coming from Asia.

Today, most paleontologists accept that primates first evolved in Asia and then made their way to Africa where they eventually evolved into the many hominid species that led to modern humans.

* Marmosets consist of 22 ‘New World’ monkey species belonging to one of four genera: Callithrix, Cebuella, Callibella, or Mico

More Analysis to Come

Some colleagues in the field have called this decade-long analysis “probably the most comprehensive phylogenetic analysis that’s ever been done on primates”. Nevertheless, much remains unknown.

Though the fossil find is remarkable for its near “completeness” — such nearly intact fossils are rare and valuable finds — the fact that its skull was crushed, leaves many questions unanswered. The team expects more debate on the exact placement of A. achilles in the primate family tree; one fossil, no matter how complete, does not in itself make for a definitive statement about primate evolution.

Beard et al are currently working on a second analysis of the fossil, but they also hope to find more like it, or related fossil species, in order to make better comparisons and thus more precisely place the creature on the primate evolutionary path.

Some source material (including quotes) for this post came from the Science NOW article:‘Crucial Link in Primate Evolution’ by Lizzie Wade

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