When a person is made, or when a mouse is made, both embryologies draw upon the same dictionary of genes: the normal vocabulary of mammalian embryologies. The difference between a person and a mouse comes out of the different orders with which the genes, drawn from that shared mammalian vocabulary, are deployed, the different places in the body where this happens and its timing. All this is under the control of particular genes whose business it is to turn other genes on, in complicated and exquisitely timed cascades. But such controlling genes constitute only a minority of the genes in the genome.
All this helps us understand why it is so easy for an expert to recognize the genome of a mammal. It has the mammal toolbox, which in addition to its specialized mammal-making tools also includes tools from the reptile toolbox, and the fish toolbox, and even the worm toolbox. The oldest tools in the kit are shared by all living things, including bacteria.
Source:
Philosopher Daniel Dennett’s Book Intuition Pumps
