When viewed within the broader context of our evolutionary history, we are anthropoid primates. That’s the group which contains monkeys and apes (with our species being a specialized variety of ape, and apes being a particular subset of monkeys, and monkeys representing the major group of anthropoids). But how anthropoid primates originated has been a subject of frequent contention.
The current consensus – on the basis of anatomy, genetics, and other lines of evidence – is that anthropoids are most closely related to tarsiers and extinct, tarsier-like primates called omomyiforms. Hence anthropoid primates probably evolved from a tarsier-like ancestor over 40 million years ago. This conclusion replaced the previous leading hypothesis that anthropoids evolved from lemur-like primates called adapiforms. As it turned out, many of the features thought to link adapiforms and anthropoids together were instances of evolutionary convergence between the prehistoric lemur-form primates and modern monkeys. The resemblances had to do with the natural history of the primates rather than a close genetic relationship.
Then Ida crashed the party. In 2009, a mega-hyped media platform announced that a 47 million year old adapiform primate found in the exquisite Messel Shale of Germany was “THE LINK” between archaic primates and the first anthropoids. This was science by press release – the paper which described and named the adapiform Darwinius masillae did not even come close to supporting the conclusions strewn through the many, many news reports on the fossil.
But the mess had at least one positive outcome. With the exception of our closest hominin relatives, fossil primates almost never make the news. Who, other than paleontologists and fossil aficionados, has ever heard of an adapiform or omomyid? The Ida kerfuffle called our attention to these lesser-known relatives of ours. One of these primates – an adapiform which climbed through the warm forests of Wyoming about 50 million years ago – is at the center of a recent PLoS One study which again puts Ida in her place.
Notharctus tenebrosus is a classic fossil primate. It was named by the Philadelphia polymath Joseph Leidy from fragmentary remains in 1870, and in 1920 American Museum of Natural History paleontologist W.K. Gregory composed a beautiful monograph which described several partial skeletons of the lemur-like animal. Despite being known for nearly a century and a half, though, we’re still learning new things about Notharctus. The new study by Stony Brook University graduate student Stephanie Maiolino and colleagues focused on the animal’s second toe and the significance of that digit for what has traditionally been thought to be a tell-tale sign of primate relationships.
One of primary facets of the Darwinius debate has been whether or not the lemur-like primate had a grooming claw. This feature is exactly what it sounds like – a pointed claw on the foot used to scratch and preen. Living strepsirrhine primates – lemurs, lorises, bush babies, and the aye-aye – all have a specialized, curved claw on their second toe used for preening. Most living anthropoid primates, by contrast, lack this feature. The fact that Darwinius seemed to lack a grooming claw on the second toe could therefore be construed as supporting evidence for a closer relationship with anthropoids than lemurs.
Alas, the situation is not so simple. A study published by Maiolino and colleagues in November last year demonstrated that owl and titi monkeys – anthropoid primates – have grooming claws, after all. And tarsiers – the closest living relatives to the anthropoid group – have grooming claws, as well. On the other side of the tree – among the closest fossil relatives of lemurs and lorises within the greater group called strepsirrhine primates – grooming claws do not seem to be a consistent feature. Some lemur-like fossil primates appear to have grooming claws, while others do not.
Notharctus was among the adapiform primates thought to have a grooming claw. As Maiolino and co-authors point out in their new study, however, this was based on an incomplete piece of the skeleton which was not found articulated with the rest of the foot. It was unclear whether this partial bone was truly part of a grooming claw.
To resolve this issue, Maiolino and collaborators studied a partially-articulated foot of Notharctus found in Wyoming sometime between 1990 and 2000. Included in the array of small bones was the second toe claw. The bone was not quite like a lemur’s grooming claw, but the bone didn’t support a typical, flat nail, either.
Notharctus tenebrosus had a second toe claw with an intermediate shape. Viewed from the top or bottom, the claw looks relatively flat, but when turned to the side the claw clearly has a steeper angle and slight curve which sets it apart from claws on neighboring toes. The bone also resembles the grooming claws of titi monkeys – anthropoid primates which have either retained or re-evolved this feature.
But was the second toe claw of Notharctus being modified into a grooming claw, or was the appendage changing from a grooming claw into a flattened toenail? The answer is unclear. We need to know whether grooming claws were an early, widely-shared characteristic which was lost multiple times, or whether grooming claws evolved – and perhaps even re-evolved – independently within different parts of the primate evolutionary tree.
For the moment, though, the work of Maiolino and co-authors has shown that grooming claws are not the unmistakable signs of affinity we previously thought them to be. Grooming claws of one form or another are found in modern lemurs and lorises, some anthropoid primates, and some lemur-like fossil primates.
So what does this mean for Darwinius? Despite the completeness of the primate fossil, paleontologists still can’t discern some of the important details of the creature’s anatomy. The million dollar fossil is embedded in shale on its side, and this makes it difficult to understand the primate’s anatomy in three dimensions. Regarding the second toe, for example, it isn’t clear whether or not Darwinius had a flattened toenail like many anthropoid primates do. Maybe Darwinius had a grooming claw like that of Notharctus – one that looks flat when viewed from above but is actually sharply angled and slightly curved.
Either way, the anatomy of the second toe claw does not appear to be especially important in parsing prehistoric primate relationships. Maiolino and co-authors ran a slew of comparative analyses to test different ideas about early primate affinities on the basis of different character codings. The researchers selected particular primates, noted the presence or absence of certain observable skeletal features, and then ran the data through a program which produced the most probable evolutionary trees on the basis of shared, specialized characteristics. In one series, the scientists tried different codings for the second toe appendages of Notharctus and Darwinius – they submitted datasets with Notharctus and Darwinius marked as possessing grooming claws and datasets with the two primates noted as lacking grooming claws to see if the differences would influence the results.
The presence or absence of the grooming claw in Notharctus and Darwinius didn’t change the shape of the results at all. And the picture that emerged was consistent with what other studies had previously found – Darwinius and Notharctus were most closely related to lemurs, lorises, and other strepsirrhine primates, far removed from anthropoid ancestry.
Although the fossil is a minute remnant of prehistory, the grooming claw of Notharctus is a testament to how much we still have to learn about the biology and evolution of the earliest primates. Archaic primates flourished in a variety of forms distributed through warm forests which once stood where cool grasslands now dominate, and in the course of evolutionary time some of these creatures converged on the anatomy of modern primates in ways we never expected. The more we learn about our distant primate cousins and forebears, the more perplexing they become.
For more on early primates and the Darwinius controversy, see:
Maiolino, S., Boyer, D., & Rosenberger, A. (2011). Morphological Correlates of the Grooming Claw in Distal Phalanges of Platyrrhines and Other Primates: A Preliminary Study The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology, 294 (12), 1975-1990 DOI: 10.1002/ar.21498
Maiolino, S., Boyer, D., Bloch, J., Gilbert, C., & Groenke, J. (2012). Evidence for a Grooming Claw in a North American Adapiform Primate: Implications for Anthropoid Origins PLoS ONE, 7 (1) DOI: 10.1371/journal.pone.0029135
Williams, B., Kay, R., & Kirk, E. (2010). New perspectives on anthropoid origins Proceedings of the National Academy of Sciences, 107 (11), 4797-4804 DOI: 10.1073/pnas.0908320107