average number of neurons in the human brain

Anumber of species have brain sizes larger than humans in absolute terms. It has long been clear that brainsize varies in a predictable way across species in relation to bodyweight, though not in a simple isometric manner. The precedingdiscussion has ignored the possible confounding influence of bodyweight. As figure 2.1 indicates,absolute brain size increased fairly steadily from this point on. The Homohabilis specimen KNM-ER 1470,which is dated to ~1.8 MYA, has a brain size estimated to be 752 cc(Holloway 1983b). The increase in brain size in hominids appearsto have begun at least by the time of Homo habilis, around 2 MYA (figure 2.1). This suggests that the common ancestor ofhumans, chimpanzees and gorillas had a brain size of somewhere close to400 cc. Wealso know that the oldest hominid species for which brain size has beenestimated, Australopithecus afarensis (dating to between 2.9 and 3.2 MYA), had abrain size of ~400 cc (Holloway 1983b see appendix A table 1).

Humans, chimpanzees, and gorillas lastshared a common ancestor between four and six million years ago, andthis group shares a common ancestor with the orangutan at about 10million years ago (Sarich 1971 Sibley and Ahlquist 1984). Bycontrast, our closest living relatives have much smaller brain sizes:Modern chimpanzees and orangutans have brains averaging about 400 cc,with gorillas averaging about 500 cc (Tobias 1971, see appendix A table3). Modern humanbrain size averages about 1400 cc (Jerison 1973 Holloway 1980 Pakkenberg 1964 Tobias 1971 Dekaban 1978 Beals 1984). Īn estimate of the change in overall size ofthe hominid brain can be derived from two sources, both of whichconverge on the same answer. That is, the human brain is not a simplescaled-up version of an ape brain, and this will give us importantclues to the possible causes of the evolution of human behavior. Furthermore,not all functional units in the human brain evolved at the same rate. There is also evidenceof changes in the relative proportions of various components, e.g., inthe amount of grey vs.

Thereis a great deal of debate over the relative contribution of structuralchanges and size increases in the evolution of human behavior, as Iwill outline below. The most obviousevolutionary change was in overall size, but it is likely that otherchanges also played an important role. We are guided both by a comparativeperspective as well as by an analysis of the fossil record('paleoneurology'). There are several sources of information thatallow us to reconstruct a general outline of human brain evolution. Chapter2: Human Neuroanatomical Evolution

Average number of neurons in the human brain