There has been a rash of books on human evolution in recent years, claiming that it was driven by art (Denis Dutton: The Art Instinct), cooking (Richard Wrangham: Catching Fire), sexual selection (Geoffrey Miller: The Mating Mind). Now, Timothy Taylor, reader in archaeology at the University of Bradford, makes a claim for technology in general and, in particular, the invention of the baby sling – not, as you may have thought, in the 1960s but more than 2m years ago.
All these theories and speculations are in truth complementary facets of an emerging Grand Universal Theory of Human Origins. The way they overlap, reinforce one another and suggest new leads is too striking to miss. What they have in common is a reversal of the received idea of evolution through natural selection. In this, a mutation takes place that happens to be useful; it is retained and spreads through the population. In the new theory, proto-human beings, through innovative technologies, created the conditions that led to a rapid spread of new mutations. In other words, we didn't evolve a big brain (three to four times the size of a chimp's) and then use it to develop human culture; we first departed from genetically fixed behaviour patterns, and this led to ever-increasing brain capacity and hence more innovations. The plethora of speculations as to how this happened is fascinating and will probably lead to a true understanding of the course of human evolution, but most people will want proof.
Impeccably detailed evidence is now emerging from the genomics revolution. Taylor cites one of the best attested examples of a human cultural innovation leading to genetic change: the drinking of cow's milk. In the ancestral human condition only babies up to the age of weaning could digest milk, but tolerance to cow's milk has spread though all populations that have practised cattle farming. Globally, this process is still incomplete and genomics has revealed that milk tolerance has evolved on several separate occasions by different genetic mechanisms.
After the switch to an upright posture, probably the biggest single anatomical change on the journey from apes to humans was the weakening of the jaw. In apes, the jaw is large and protrudes way beyond the nose. It is attached by muscle to a bony ridge on the top of the skull and has a force many times that of a human jaw. Recent genomics research has shown that a large mutation about 2.4m years ago disabled the key muscle protein in human jaws. We still have the disabled protein today, and that weakened jaw enabled a raft of innovations. The ape brain could not grow because of the huge muscle load anchored to the skull's crest, and apes cannot articulate speech-like sounds because of the clumsy force of their jaws. This mutation allowed the increase in human brain size and the acquisition of language.
But why did it happen? Wrangham maintains that it was cooking that led to the change. Cooked food does not need strong jaws. In genetics a function that becomes redundant always leads to the gene being disabled by mutations. Around 2.4m years ago an ape switched to mostly cooked food. In the fossil record, a new proto-human appeared 1.8-1.9m years ago: Homo erectus had a much larger brain and no crest on the skull, indicating that the weakened jaw muscle was now standard.
There were other advantages to cooked food. It seems that in all animals the gut and the brain compete for energy: creatures with large guts spend many hours a day eating and have small brains. Humans have a gut only 60% as big as you'd expect for their body size: cooked food made that possible, and the energy saved went into feeding that enormous brain.
Taylor endorses Wrangham's hypothesis but believes it is not enough. Not only is our brain very large, it is proportionately enormous at birth, creating problems at delivery for narrow-hipped, upright-standing women and even more during the first few years, when babies are extremely vulnerable. Factor in the African savannah 2m years ago, teeming with enormous predators, and you wonder how we are still here. For Taylor, the crucial innovation was the baby sling, which enabled proto-human mothers to carry their vulnerable babies (infant apes, of course, cling to their hairy mothers' backs).
Unlike milk tolerance, jaw muscles and gut length – all amenable to genetic investigation in the present – prehistoric baby slings have left no evidence behind, so this hypothesis is likely to remain speculative. For the lack of any clinching evidence, Taylor allows himself to be side-tracked in the second half of the book into Barthesian digressions on the role of the object in human cultures. Some of this material is far-fetched, reaching its nadir in the suggestion that in the mirrors given to them by French sailors in 1772, the doomed Tasmanian Aborigines saw "some premonition of the coming global age of screen culture".
This loss of focus is a pity because Taylor, along with the other writers mentioned, is clearly on to something. The new understanding of human evolution should be a massive relief to many. The anguish that Darwin caused – all purpose gone, chance and brute necessity rule – seems to be have been misplaced. There is no goal in nature, nor any God-given purpose, but human evolution has been driven by striving towards a better way of living. As they domesticated cattle, sheep, goats, pigs, horses, cats, dogs and bees, humans were simultaneously domesticating themselves. By our own efforts we made ourselves human.
Peter Forbes's Dazzled and Deceived: Mimicry and Camouflage is published by Yale.