What makes us human?

In one of the articles collected by the British biologist Julian Huxley (1887-1975) in his book Man in the Modern World, titled The uniqueness of man, he asserted: «Conceptual thought on this planet is inevitably associated with a particular type of Primate body and Primate brain» (p.16). This assertion may seem determinist, but it does not mean that the appearance of our species (and its distinctive quality, conceptual thought) was inevitable, rather, that it was only possible for specific group of mammals, primates. This Room, with the title ‘What makes us human?’, takes on understanding our species, Homo sapiens, in a mammal and primate evolutionary context, focalising our long biological history—120 million years from the moment the first mammals appeared— on the evolution of the life cycles and life strategies of our ancestors, from the remotest, the first primates, to our brother lineages, the Neanderthals.

The Room revolves around two considerations by USA anthropologists Brian T. Shea and Catherine A. Key: the first is that we primates are very special mammals; the second is that we human beings are extreme primates. It is our life stories which make us so peculiar. We primates are radically precocial mammals, so that we reproduce at a rate which is half that of any other mammals with a similar body size, and our offspring —one or two at most— require a long period of parental care because they also grow at half the rate of the other mammals. The American biologist Eric L. Charnov therefore described us as «atypical mammals», as it would seem that we avoid the constants which regulate the relation between body size and metabolic energy in living beings. We will evaluate whether our uniqueness among mammals is due to our relatively large brains (a large brain in relation to body size which is called «encephalisation»), reviewing to this purpose our evolutionary history and seeing which orders of mammals are phylogenetically closest to ours, and whether their characteristics shed any light on our particularity. Then we shall reflect firstly on the evolution of the life cycle in our hominin group (biped hominids), on when and how our own life cycle appears in our species, reviewing the evidence from fossil registers about the life cycle of our ancestors, and secondly, we shall consider whether the peculiarities of our life cycle (how long it is and the new stages involved) derive from our extreme encephalisation.

Finally, we shall see how the characteristics of our life cycle (together with our cultural capacity) have allowed us to adapt plastically to new and different ecosystems during our planetary expansion as Homo sapiens, from 100,000 years ago. [Carlos Varea]

[The four Galleries of this Room are available in Spanish at ¿Qué nos hace humanos?]