Wednesday, 28 January 2015

New Evidence Indicates That The First Primates Were Arboreal

In 2008 the fossil of a primitive 47 million year old primate, nicknamed Ida, caused a stir as early examinations of its anatomy suggested that it might be the common ancestors of all primates. However, these early interpretations were shown to be largely incorrect. Ida was an early form of lemur rather than the common ancestor of all primates. It is likely that media hype, combined with the fact that it was discovered in the bicentenary of Charles Darwin's birth, led to its erroneous celebration. Since then fossils have shown that primates evolved millions of years before Ida. Yet how and why they evolved remained a contentious issue.

Plesiadapiformes have a shrew like morphology, but their teeth
link them to the primates and their ankles to an arboreal lifestyle
The earliest mammals were ground dwellers, but it is thought that the primates evolved on the ground first before making a move into the trees. Today most species of primate are arboreal.

New fossil evidence has shown that the group may actually have started out life in the trees before the lineage leading to chimps, apes and ultimately us, returned to the ground. Between 66 and 63 million years ago there lived a group of mammals known as the plesiadapiformes from which the primates are thought to have evolved. One of these, a species called Purgatorius, is thought to sit very close to the common ancestor of all primates. This gives us an excellent indication as to what the first members of the group looked like and how they may have lived.

Analysis of 65 million year old ankle bones from Hell Creek in Montana suggests that Purgatorius and the plesiadapiformes were arboreal, meaning that the first primates would also have been arboreal. 'The ankle bones have diagnostic features for mobility that are only present in those of primates and their close relatives today,' said Stephen Chester, a doctoral student at the University of New York. 'These unique features would have allowed an animal such as Purgatorius to rotate and adjust its feet accordingly to grab branches while moving through trees. In contrast, ground-dwelling mammals lack these features and are better suited for propelling themselves forward in a more restricted, fore-and-aft motion.'

An artist's impression of Purgatorius in its arboreal habitat
Previous studies of the teeth and diets of plesiadapiformes suggest that they fed on fruits and seeds. An arboreal lifestyle would have been more suited to supporting this diet rather than a terrestrial existence relying on fruit-fall from trees. Similarly, it would have offered some protection against predators.

All in all the evidence points towards an arboreal origin prior to the evolution of the primate family themselves. What this implies is that the evolution of the primates was not a rapid diversification, as would be the case if they moved into the trees shortly after the extinction of the dinosaurs 65 million years ago. Instead evolution resulted from small changes in an already arboreal group of mammals. Yet this lifestyle is just one part of the story. Primates owe their success to their gregarious nature and problem solving skills, rivaled by few other creatures.