Hand Axes as an Extended Phenotype

Two and a half million years ago, our small-brained ancestors evolved the ability to knock flakes from rocks to use as cutting edges. By doing so, they could also make the rocks themselves useful as choppers. This basic tool kit of flakes and choppers served the needs of hunting and gathering for a million years. Then, around 1.6 million years ago, a medium-brained African hominid (Homo erectus) evolved the ability to produce an extraordinary object that archeologists call a handaxe. A handaxe is a rock chipped into roughly the size and shape of a child's hand—flat with fingers together. There is a sharp edge all around and a point at the tip. The outline is midway between that of a pear and a triangle. The top and bottom faces are symmetrical (handaxes are also called "bifaces"), as are the right and left halves. Most were made of flint, some of quartzite or obsidian.

Handaxes proved enormously popular. They were made for over a million years, until about 200,000 years ago, by which time our ancestors had evolved into large-brained archaic Homo sapiens. Handaxes were made throughout Africa, Europe, and Asia, and in enormous numbers: sometimes hundreds are found at a single dig site. The persistence of a single design across such a span of space and time cannot be explained through cultural imitation. Designs passed down through mere imitation tend to deviate further and further from the original prototype, as languages do over hundreds of years. Handaxes must have been to hominids what bowers are to bowerbirds: part of their extended phenotype, a genetically inherited propensity to construct a certain type of object.

If, as the author assumes, handaxes were genetically-informed because they did not change for hundreds of thousands of years, then why do we not still have the instinct for hand axes?