Among other things, I’m a primate. Whether I can call myself a fish or an ape is a matter of semantic dispute, as last month’s wide-ranging blog discussion showed, but there’s no real controversy in identifying myself as a member of the wider primate family. When primates originated, though, is a rather thorny problem.
The fossil record is the most direct way to track when lineages first evolved. By searching for the earliest members of a group, and comparing those organisms to those already known, paleontologists can get a handle on when that group originated. An archaic form with few specializations was probably close to when that lineage became established, but if the earliest known representative of a group is already highly-differentiated, then there must be an as-yet-undiscovered “ghost lineage” which connects that group to some earlier ancestor.
The earliest primate fossils from members of the crown group – the category which contains the haplorhines (tarsiers, anthropoids, and their closest relatives) and strespsirrhines (lemurs, lorises, bush babies, and their closest relatives) – are around 56 million years old. (Specifically, the earliest known crown group primate is Teilhardina asiatica.) This hints that primates were one of the many mammal lineages which proliferated in the wake of the end-Cretaceous mass extinction which wiped out the pterosaurs, mosasaurs, ammonites, and various other forms of life, including the beloved non-avian dinosaurs.
But another methodology has yielded conflicting results. Living creatures carry records of their evolutionary past in their bodies, and molecular biologists have tried to use differences between living organisms at the level of genes and proteins to estimate when those lineages last shared a common ancestor. This is popularly known as a molecular clock. By using fossils as calibrating points and calculating how much time it would take for differences to accumulate given an estimated mutation rate, these studies have suggested that primates actually originated before the Cretaceous mass extinction, around 82 million years ago. If these estimates are right, then lemur-like primates might have hidden from feathered, sickle-clawed deinonychosaurs in Cretaceous forests, although the fossil record has not confirmed this.
Sometimes, molecular clocks and fossils are in general accord. Estimates for when the first humans originated were confirmed to be in the right ballpark thanks to discoveries of early humans in the 4-6 million year range. But in the case of primates in general, the fossil record and the genetic record don’t match.
An interdisciplinary PNAS paper has arrived at a compromise between the two conflicting sides. According to anthropologists Michael Steiper and Erik Seiffert, the anatomy of the earliest known primates might elucidate a pattern that has thrown modern molecular estimates off the mark. If they are right, primates originated right around the boundary between the Cretaceous and the Paleocene – primates probably emerged into a world recently vacated by the non-avian dinosaurs.
Early molecular clocks relied on constant rates of change for their estimations. When the rate of change fluctuates, though, things get sticky. Scientists have attempted to address this problem by using fossils to calibrate timelines and anticipate times of faster or slower change, but the discord about primate origins has remained.
According to Steiper and Seiffert, though, previous origin estimates were thrown off by a gradual slowdown of molecular change among fossil primates. There’s no way to detect this directly – we can’t study the DNA or proteins of the earliest primates – but three skeletal traits might be signals of how fast or slow molecular rates of change were progressing. The traits are body size, absolute endocranial volume (a proxy for brain size), and relative endocranial volume. These features, the anthropologists argue, all influence primate life histories – how long they live, how quickly they reproduce – and so would have affected rates of molecular change. By tracking these traits, studied in the context of how they affect the natural history of modern primates, we might be able to develop a more precise picture of early primate evolution.
Based on the available fossils and their models, Steiper and Seiffert write that the earliest crown group primates were probably very small and had tiny brains. Imagine something the size of a mouse lemur. Among these small primates – which probably reproduced rapidly – differences in genes and proteins quickly racked up, an idea supported by studies of how body and brain size affect the rates of molecular change. But as primate body and brain size increased – in a proliferation of bigger, longer-living species – the rate of molecular change slowed in both haplorhine and strepsirrhine primates. When this previously undocumented lag is accounted for, then the molecular estimate for the origin of crown group primates is about 65 million years ago – a figure closer to what the fossil record indicates. The end-Cretaceous extinction made the origin of our own family possible.
The same method might be useful in refining estimates for other groups, too. For years, paleontologists and molecular biologists have been going back and forth over the origin of many mammal groups – whether mammals proliferated after the end-Cretaceous extinction, or whether most modern mammal lineages appeared while tyrannosaurs and ceratopsids still roamed. The technique proposed by Steiper and Seiffert moves beyond simply using fossils as waypoints and instead looks for clues within the natural history of extinct creatures which can tell us something about the rate of molecular change. By using fossils, studies of modern animals, and molecular biology, scientists may be better able to fill in key dates on evolution’s calendar.
Steiper, M., & Seiffert, E. (2012). Evidence for a convergent slowdown in primate molecular rates and its implications for the timing of early primate evolution Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1119506109