The Evolution of Intelligence

Renato M.E. Sabbatini, PhD
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Brain sizes can be estimated from the internal volume of fossil skulls. When a corrective formula for body size is used, average brain size is a good indicator of relative intelligence. The overall brain body ratio of non-human primates is above that of vertebrates as a whole. The brain body ratio of Homo compared to that of the non-human primates is even greater. The following graph shows that all primates brain/body ratios lie on or above the regression line (in magenta). New World monkeys, such as marmosets, are much less intelligent than Old World monkeys, such as macaques and baboons. Chimpanzees, orangutans, gorillas and early hominids (Australopithecus afarensis) have about the same brain/body ratio. Homo ergaster and Homo sapiens lead the crowd.

 

The evolution of brain size in hominids and Homo is striking. In the period of 3,5 million years it increased almost fourfold, following a mild exponential curve. The trend can be seen in the following graph from McHenry (1994), plotting brain sizes against time, shows a general trend towards increased brain size over time for the hominids and Homo species:
 

As we can see, the Australopithecines, which appeared 3.5 to 3 million years ago, had a brain three to four times smaller than modern humans (on the average, 450 cubic centimeters versus 1,350 cc). Thus, it seems that they were slightly more intelligent than apes, although they probably already used crude tools and had upright position and were bipedal. This was proven by the discovery of paths of fossil footprints left by Australopithecus afarensis, as well as by the analysis of its skeletons.

Then, a sharp increase in brain size occurred about 2 to 1.5 million years ago by the appearance of Homo habilis, going to around 650 to 800 cc. This species was suceeded by Homo erectus, who lived between 1,6 million and 300,000 years ago and had a cranial capacity of 850 to 1,000 cc. Thus, unlike the lines which precede them, H. erectus is considered to have been not just human-like, but human. Evidence of growing brain size and the ability of Homo erectus to walk upright is convincing enough to place this species immediately before Homo sapiens but after Homo habilis on the evolutionary chain. Their brains remained relatively stable for a very long period, in the so-called "stasis period"). Then, from 600,000 to 150,000 years ago, the brain of Homo increased very sharply again, reaching 1,200 to 1,350 cc (Homo sapiens). The brains of Neanderthals were larger than those of modern humans (average of 1,500 cc), but this is argued to be the result of a larger body mass.

Another indicator of intelligence is the total area and the degree of complexity of the brain cortex. It can be estimated from endocasts (impressions left by the cortex convolutions on the internal surfaces of the cranium.. Tool marks on animal bones found near the hominid fossils indicate that H. habilis ate meat. We don't know whether he had the capability of individual or group hunting, but since even chimpanzees show some form of hunting small animals for food, it is possible that they did it.

Homo habilis had a much larger brain size than Australopithecines. Some studies of the endocasts of H.habilis skulls indicate that the prefrontal region of cortex which is essential to speech was already developed in this species, so probably it had some sort of crude communication language based on vocal articulations. On the average, Australopithecines were only as smart, or slightly more as modern chimpanzees. In a sense, they were no more than upright-walking apes. Homo habilis and erectus were probably intermediate in intelligence between chimpanzees and modern humans.

How early hominids appeared?

Australopithecine ape-men probably evolved from one of the estimated 20 different species of forest-dwelling apes living in East Africa 10 million years ago, and were the result of drastic geological and climate changes at the end of the Miocene period, which was marked by a long dry spell in equatorial Africa. The Miocene forest shrank considerably in size and the African savannah developed, leading to the extinction of many species, and breeding others, which became the home for many modern African animals.

The absence of large trees and forests forced the evolution of "habitual bidepality" on this tree-dwelling ape, long before it evolved articulate language and other hominid features. The earliest known species of hominids, Australopithecus anamensis, dated to more than 4 million years, had human-like bones of the leg and pelvis, adapted for erect posture and bipedal locomotion, although their crania, mandible and teeth were still very much ape-like. In any case, these earlier hominids were still not human, because they showed none of the cerebral expansion and little of the dental morphology which would characterize later hominids, such as Homo habilis.

Many scientists believe that "opportunistic hunting" on the African savannah, or veld, was the most important factor for the development human bipedality. Why? Because, by exploring the savannah, proto-hominids would encounter food sources in the form of carrion left by the predators, as well as young or sick animals. In order to carry this food back to their living places and social group, it required free hands and an upright locomotion for long distances, because humans do not have strong jaws and teeth like lions or hyenas. This hypothesis, first formulated by Owen Lovejoy, explains also how carrying food and sharing it with the social group eventually led to the family nucleus and the development of different roles for man and woman, which is so central in our reproductive biology. This allowed females to procreate more often and have a much larger offspring than chimpanzees, causing a more rapid population expansion.

The Author

"The Evolution of Intelligence" Renato M.E. Sabbatini, PhD Brain & Mind Magazine, February/April 2001