by Jonathan Kingdon
Subtitled “Where, When, and Why Our Ancestors First Stood Up”, this book explores the entirety of hominid evolution. The author is a researcher at Oxford, and spent a goodly portion of his life in east Africa, where hominids first appeared. He presents a theory – not just a single hypothesis, but a long, detailed narrative of how hominids developed. I suppose that some people would call this a “just so story”, but I call it a theory.
His version of the story is complicated. The key to the early evolution of hominids, he believes lay in the combination of two factors. First comes the set of river valleys along the coast of east Africa. These all run a short distance – perhaps a hundred miles – from the highlands down to the sea in an east-southeast direction. Each valley is a fertile region with lots of sources of food. Kingdon believes that primates in these valleys flourished in the trees, and some extended their abilities to gather food on the ground. However, they never lost their facility for zipping up trees when danger threatened. This was the starting point for hominid evolution.
The second crucial factor was the waxing and waning of warm climate. When it was warm, the tropical forests in the river valleys spread out into the regions between them, linking all the river valleys into a single ecosystem. In these circumstances, the various primate species tended to pool their genetic resources, increasing the size of the gene pool. But the warm, moist periods always came to an end, to be replaced by cool, dry climates in which the tropical forests retreated to the river valleys, isolating the species’ populations.
During these retreating phases, the primates who had developed ground-based behaviors extended those behaviors. In particular, they developed a system of scouring the leaf litter under trees for loose nuts, berries, and fruit. This required them to squat on their hind legs and use their forelegs to riffle through the leaf litter, searching for food. They’d steadily move forward, using their hind legs for support and their forelegs for searching. This led to increased emphasis on hind legs for locomotion and forelegs for manipulation. Primates who live exclusively in the trees use all four limbs for locomotion, but these new ground-based primates shifted their postures towards a lopsided use of their limbs. This was the beginning of bipedalism.
Again, they never lost the ability to scamper up trees when necessary; this was their only defense against predators. But each time the forests shrank, they were forced to forage in more open areas. The greater separations between trees required them to develop a better means of traversing these greater distances, which pushed them further along the path to bipedalism. The end result of these cycles of warming and cooling was a fully bipedal group of hominid species.
Kingdon emphasizes that we should not think in terms of a single evolutionary path leading straight to Homo Sapiens. Instead, there was a goodly variety of hominid species. We know of at least a dozen distinct species in the genera, but there’s still lots of disagreement about the structure of the evolutionary tree on which they lie. Kingdon has his own opinions on that tree, but freely acknowledges alternative views.
The next stage in hominid development was something that Kingdon calls “niche stealing”. The bipedal hominids were able to use their hands and larger brains to exploit other food sources, and so were able to take over ecological niches previously reserved to specialist animals. They developed schemes for gathering shallow-water food sources, various insect sources, and root foods. This brings us up to Homo Erectus, which was so successful at niche stealing that it broke out of Africa and spread along the southern coast of Asia, then the eastern coast all the way to China. In the process, Erectus developed into even more advanced forms, learning how to build primitive boats, which greatly expanded their food sources. This led to a counter-invasion, in which the new improved Erectus repopulated Africa, replacing the hominids who had stayed behind. Back in Africa, there were further developments which I won’t try to explain. One of the subsequent species was Homo Neanderthalis, which broke out of Africa about 500,000 years ago and specialized for the cold climates of northern Eurasia. Shortly afterwards, perhaps 300,000 years ago, Homo Sapiens arose and it too broke out of Africa, eventually wiping out all the other hominid species.
That’s the short version of the story told by Kingdon. It is backed up with much detail from the library of fossils that have been found over the years. This is certainly the most thorough, most detailed, most heavily substantiated version of human evolution that I’ve ever read.