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The Improbable Path to Advanced Intelligence

Astronomers interested in extraterrestrial intelligence often argue “Improbable though the evolution of advanced intelligence may be, you must admit that there is only one sample, one planetary history and that intelligence evolved on earth.” But this is naïve. Biogeography and paleontology show that earth provides not one sample, but many millions of samples. Sketches of a handful of these samples follow.

Placental mammals never made it to Australia. Instead, the marsupials radiated there into a very wide variety of niches that remarkably paralleled their mammalian lookalikes elsewhere: marsupial dogs, cats, moles, flying squirrels, and kangaroo mice. Of course some of the niches in Australia were filled by unique adaptations. The kangaroo, for example, fills the grazing/browsing niches occupied by ungulates elsewhere. Not only did intelligence, or even placentation, fail to evolve among these marsupials (who after all covered a continent the size of the United States), but a form even remotely like our primitive primate ancestors also failed to appear. From the earliest days the homeland of the entire primate lineage has been in the trees, and it was not as if the “tree niche” in Australia had not been exploited by marsupials. The koala’s adaptation to tree life makes it a marsupial’s nomination for a monkey. If you’ve ever held a koala in your arms, you know that its brain and behavior could only be generously described as “somnolent.” The other tree-exploiting candidate for primatehood in Australia is the tree kangaroo. If ever there were a hopeful monster, this must be it. Its secondary adaptation to tree life was cobbled together from its hopping adaptation to the ground. So long as there are no serious predators, the tree kangaroo can make comfortable living in the branches. But when humans appear it becomes just another easy lunch. The Australian fauna illustrate a remarkable independent radiation of forms parallel to the mammals. But nowhere do these marsupials display any inclination toward advanced intelligence, or even important preconditions for it to develop.

Consider another example, the island of Madagascar. Some fifty percent of all primate varieties live on Madagascar, but they are all prosimians. Madagascar broke away from Africa before monkeys colonized it, and no monkey or ape has ever emerged on Madagascar.

This does not mean that prosimians were not successful. Far from it, they radiated into an enormous variety of niches. There are prosimians adapted to swamps, deserts, mountain forests; they are mostly found in trees, but some adapted to the ground and all habitats in between. Like other prosimians in Africa and Asia the prosimian’s body and brain reflect the fact that they live in a world in which tactile and olfactory reception is as important as vision. Most have vibrissae, scent glands, and a basic mammalian body plan. Some have multiple young and some forage at night. Yet they also have the primate hallmark: grasping hands. Prosimians include some of the smallest mammals on earth, and in historic times there were lemurs as large as a donkey. Despite this rather astonishing radiation of forms and adaptations, prosimians never evolved what could even be generously called a “monkey-level of intelligence.”

The tarsier, a prosimian of Asia, developed binocular color vision rather like the system that evolved in humans. Why should this be when other prosimians lack binocular vision. The tarsier is about the size of a rat; it bounds around in trees in semi-darkness in the Phillippines and parts of southeast Asia. A unique trait in this group is the greatly elongated tarsial bone, which gives the tarsiers a great deal of extra leverage when they decide to hop. If humans had such an adaptation, we could all leap tall buildings at a single bound. During a long leap a tarsier can grab a moth in mid-air and still make a safe landing. Such acrobatics require the precision provided by stereoscopic color vision. It is ironic (but no great surprise in evolutionary history) that a characteristic like binocular color vision, that turns out to be so critically important in the development of our own intelligence and technology, also evolved millions of years ago in an utterly different context, and in support of a very different evolutionary path.

The evolution of monkeys provides two parallel examples. After South America split away from Africa, the independent evolution of New World monkeys reveals many close parallels with Old World monkeys, but significant differences as well. South American monkeys have exploited a wide variety of niches. There are many species of squirrel-sized, pair-bonding (in fact, often polyandrous) marmosets and tamarins. There are species with male-pattern baldness and no tails. Provocatively, the “ape niche” has been filled in South America by three genera of large monkeys, but in a manner utterly different from arm-swinging apes. Instead these monkeys developed a fifth and unique appendage, complete with fingerprints: the grasping tail. Their prehensile tails allow them to distribute their weight and harvest fruits and other goodies out on the terminal limbs of trees where the foraging is richest. So, the ape niche was filled in the South American rainforests, but not by an ape. Indeed, there is no reason to suppose that a creature the size and intelligence of the great apes would ever have appeared in South America.

In numbers and variety the Old World monkeys, found throughout Africa, tropical Asia and southeast Asia, are among the most successful of all mammalian species. The baboons of Africa and their close cousins, the macaques of Asia (e.g., the rhesus monkey) are well known to most people. I began studying baboons in 1959Irven DeVore, ed., Primate Behavior: Field Studies of Monkeys and Apes, (New York: Holt, Rinehart & Winston, 1965).because, quite unlike most monkeys and apes, baboons foraged on the ground in large multi-male groups, were reasonably good hunters, adroit at escaping predators, and had a social organization that could support a long period of infant dependency. In other words these large monkeys show many of the traits we know became important in hominid evolution. (But such comparisons can be easily stretched too far; baboon solutions to their terrestrial niche are still “monkey solutions,” and quite unlike the way of life, strategies, etc. we know from human foragers).Richard B. Lee and Irven DeVore, Man the Hunter, (Chicago, IL: Aldine, 1968).Baboons seem to have many of the prerequisites for advanced intelligence: large brains, intense curiosity, and exercise considerable insight and guile in social interactions. From their long fossil record we also know that there have been many species of baboons, some quite small, and some gorilla-sized. Despite their admirable successes - their ingenuity, social complexity, and long lives - as far back in prehistory as we can trace baboons, they seem to be built to the same basic plan. From the point of view of developing an ape or human level of intelligence, one can only say that baboons are mired in a baboon niche.

Contributed by: Dr. Irven DeVore

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The Improbable Path to Advanced Intelligence

Is there other Life in the Universe?
Could E.T. Call Home?
Chance, Competition, and Catastrophe
The Rise and Near Extinction of Early Primates
The Origins of Human Intelligence
The Hominid Lineage


Irven DeVore

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