The Rise and Near Extinction of Early Primates

The history of evolution, like all history, is full of quirks, near escapes, sudden successes, and other unpredictable events. The earliest of our primate ancestors were among the earliest and most numerous of all mammals, dating from the beginning of the Tertiary, the “age of mammals,” some sixty million years ago. One prominent candidate for “earliest primate ancestor” is a peculiar proto-primate that looks more like a rodent than a primate; it has large incisor teeth for gnawing, and claws rather than fingernails. If you passed one in a city park, you might well guess that you are looking at a kind of squirrel that you hadn’t noticed before.

Although the earliest primates had a very promising start in the early Tertiary, they soon began to shrink dramatically in numbers. Not coincidentally, rodents, who appeared later in the fossil record, entered earth’s history with an awesome radiation of their own - exploding into terrestrial environments throughout the world, becoming, along with bats, the most numerous mammal species. The rodents were a serious threat to the survival of the early primates. The early primates, living in forests (as do most primates today), were at a tremendous disadvantage competing with rodents for forage in the trees. Primates have only one set of adult teeth, and as gnawers they were seriously challenged by the rodent adaptation of growing incisor teeth throughout their lives.

The survivors from these early primates were atypical - they developed grasping hands and feet. With this adaptation even large primates, by using four grasping appendages, could distribute their body weight among the branches, and forage among the smaller terminal branches of a tree, harvesting fruits, nuts, and fresh leaves more successfully than much smaller squirrels could manage. So in many parts of forest ecosystems, grasping appendages could successfully trump continuously growing teeth. Uniquely among mammals, all members of the Order Primates have grasping appendages (usually all four) with fingernails (whose histology is anatomically quite distinct from that of typical mammalian claws). The grasping hand is a singular primate adaptation, and a symbol of the whole Primate Order, found in prosimians, monkeys, apes, and humans.

To illustrate the difficulty of predicting adaptations and “advances” from the fossil record, consider the ceolacanth. The ceolacanth is a living representative of an ancient group of lobe-finned fishes, the crossopterygians, who were believed to have been extinct for 65 - 70 million years. They are of special interest to paleontologists because they appear to be the best candidate for a creature that was a transition stage from fish to amphibian. The thought was, to caricature it, that ceolacanths and their kin were lurching along on stubby fins from puddle to pond, in the hope that their offspring might someday be amphibious. But when a live ceolacanth was unexpectedly caught in 1938, it was not mucking around in brackish ponds, intent on being a proto-amphibian. Instead, its habitat is now known to be 200 feet down on a coral reef; no one has a good idea why its stubby, powerful fins are an advantage down there.

“Romer’s rule” summarizes arguments such as the above succinctly. Alfred S. Romer was a famous evolutionary biologist who wrote the “bible” on the evolution of the vertebrates. His specialty was the fossil history of the amphibians. Romer famously said that an amphibian is not a deliberate transition between fish and reptile at all; an amphibian is not trying to conquer the land, but is so desperately trying to remain a fish that it is willing to slog from pond to puddle in order to lay its eggs in water and allow its babies to grow up like proper fish. It follows that the initial survival value of a favorable innovation is conservative, in that it attempts to maintain a traditional way of life under altered circumstances. In this sense biological innovation that results in the emergence of new forms does so in spite of every effort by the species to sustain itself in its present niche.

To summarize, I believe that the enormous array of contingencies leading to ourselves is such that, if an ancient lobe-finned fish had swum up the south rather than the north fork of an estuary in ancient Gondwana, we would not be here.

Contributed by: Dr. Irven DeVore