Technology thwarted my posting of this yesterday, so here it is, a bit late…
Embryology may not be your classic hungover-after-Jubilee-weekend-chat subject, but after it came up this weekend, it reminded me how fascinating I find it, so I thought I’d share a tiny taster of it with you.
Humans (alongside all other things with backbones, the echinoderms like starfish, and some other odd looking beasties called acorn worms) are part of a superphylum called deuterostomes. Oh science does love a bit of Greek to name things. The ‘deutero-‘ part means ‘second’ and ‘-stome’ comes from the word for ‘mouth’. So basically ‘second mouth’. Why are deuterostomes called this? It’s all down to how the embryos of the species in this superphylum develop.
After two gametes join to form a zygote, the cells divide until a hollow ball of cells called a blastula forms. Eventually the cells begin to invaginate in to the centre, forming the blastopore. It is the fate of the blastopore that gives us the ‘second mouth’ name. In protostomes, the sister group to deuterostomes (their name means ‘first mouth’), the blastopore becomes the mouth – hence ‘first mouth’ – see what they did there? In deuterostomes, the blastopore forms the anus, and a secondary invagination forms the mouth, with the gut forming to connect them.
This key difference in embryonic development is what defines these two groups, and has now also been backed up by genetic studies of the genes that control the movements of the cells in the blastula. And it was a split that occurred hundreds of millions of years ago.