The Caveman’s Corner

Anthropoid Origins – A Primer

Anthropoid origins is likely the single subject dearest to my heart in all of science, so I thought I might want to lay out in a series of essays for the reader some of the basic controversies surrounding this fascinating but much under-covered topic. I’d also like the sharpen up my own knowledge a bit as I fear it has been slipping. I’ll begin here by laying some of the basic concepts and terminology of the debate.

Anthropoid diversity

First, what is an anthropoid? “Anthropoid” is a term that has suffered somewhat from a past history of loose application. You might be familiar with the term “anthropoid ape.” Insensibly, at one point this was another way of saying “great ape,” referring to members of the family Pongidae. The term still makes appearance in Ernst Mayr’s What Evolution Is, but thankfully for general comprehension it seems to have fallen entirely out of modern primatological literature.

In the true taxonomic sense, an anthropoid, or sometimes simian, is any member of the superfamily Anthropoidae of the Order Primates. These are monkeys, apes, and humans. An anthropoid is an anthropoid because of its collection of advanced features: a large brain, reduced snout, thick jaw and high face, the bony wall enclosing its eye socket, and its heavily forward facing eyes.

Cercopithecus mona, a characteristic anthropoid.

Now there are two ways of splitting up the Order Primates, and both are relevant to anthropoid origins. The traditional division sets up two “grades” based on a rough feeling of evolutionary development. The Anthropoidae is the more advanced of these grades, the other is the Prosimii, which incorporates the rest of the Primates (lemurs, bushbabies, tarsiers, etc).

Valecia veriegata, a characteristic prosimian.

Primatology, like all the rest of biology, has felt the impact of the cladisitic revolution and there is an alternate cladistic taxonomy. It sets up two sub-orders with support originating from molecular data, some cranial features (more on that later), the structure of the eye, and nominally the moisture of the external nose. The Hapolorhini are the dry-nosed primates (anthropoids and tarsiers) while the Strepsirhini are the wet-nosed primates (prosimians minus tarsiers).

Schematic representation of the taxonomies.

So why, you ask, is the debate about anthropoid origins and not haplorhine origins? Well, anthropoids are still a monophyletic clade under the cladistic system, and it is the development of many of the relatively unique features which distinguished the anthropoids as a grade in the first place which interests so many researchers. Also, the traits which distiguish the living Haplorhini and Strepsirhini are often difficult to infer or absent in fossil species. The structure of the nose is an obvious example. All living Strepsirhines also retain a claw and a row of lower teeth known as a tooth comb used in grooming which are missing in the major early prosimian groups. Thus “anthropoid” and “prosimian” have a utility for the fossil record that “haplorhine” and “strepsirhine,” anchored in modern characteristics, lack. The possible origin of the Haplorhini as a whole does play importantly into understanding the evolution of the anthropoids particularly, though, so rest assured the question is not ignored.

Features distinguishing living strepsirhines missing in fossil prosimians.

There are four competing models seeking to identify the ancestry of the anthropoids. I will briefly give them hear as a sort of table of contents for the succeeding essays:

Adapoid origins – The hypothesis that anthropoids descended from relatively advanced members of a group of Eocene prosimians known as the Adapoidae.

Omomyoid origins – The hypothesis that anthropoids descended from relatively advanced members of another group of Eocene prosimians known as the Omomyoidae.

Tarsier origins – The hypothesis that anthropoids descended from tarsiers.

Deep origins (or the “ghost lineage” theory) – The hypothesis that anthropoids share a common ancestor with the tarsiers far back into the early Eocene or late Paleocene.

Finally I’d like to try to write something up on the various models that seek to explain the functional reasons for certain characteristic aspects of anthropoid morphology, like our enclosed eye sockets. Then, as a postscript, I’ll give an account of the anthropoid fossil record from the Oligocene up when the major groups of today’s living primates began to diversify.

The resources on the internet for this are a bit barer than I expected. For more refer to Fleagle’s excellent Primate Adapatation and Evolution as well as almost any introductory text on primate anatomy or evolution.