What Makes a Human?

Sometimes it is hard to see the forest for the trees. Only 1.2% of the human genome's three billion base pairs encodes proteins. Recent estimates peg the mammalian protein-coding gene count at about 20,000–25,000, similar to other vertebrates and barely more than Caenorhabditis elegans (19,000), a simple, 1,000-cell nematode. Yet in humans these proteins help build a complex organism of nearly 100 trillion cells precisely arranged into many different organs and structures.

Where then are the instructions for building so complex an organism? Where, too, are the differences between humans and other species encoded? Perhaps it isn't the gene count, per se, that matters. After all, intricate objects like aircraft are often built by assembling small numbers of primary components into ever more complex modules. In such endeavors, assembly plans and control systems are at least as important as the components themselves.

Not surprisingly perhaps, the proportion of required ontological ...