Humans and starfish share an embryo that builds its bottom first
In a human embryo, as in a sea urchin's, the very first opening to form does not become the mouth. It becomes the anus, and the mouth breaks through later on the far side. That odd order gives the group its name, deuterostomes, Greek for second mouth, and links vertebrates to starfish and acorn worms.
Deuterostomes are one of the two great branches of animals with bilateral symmetry. They include the chordates, which contain all vertebrates, the echinoderms such as sea stars and sea urchins, and the hemichordates such as acorn worms. Their sister branch, the protostomes, form the mouth first and include insects, molluscs and most worms. The anus-first rule has since turned up in some protostomes too, so it no longer defines the group on its own; deuterostomes are also called enterocoelomates, because their body cavity forms from pouches budding off the gut.
Their early embryos have a striking property. Most show indeterminate cleavage, meaning a cell's fate is not fixed by the cell it came from. Separate the first four cells and each can grow into a complete small larva, and if one cell is removed from the early ball the others make up for it. This flexibility is what allows identical twins.
Membership has changed. Groups such as the lamp shells and the arrow worms were once counted as deuterostomes on anatomical grounds, but DNA analyses from 1995 onward moved them among the protostomes. Genetic work has also found more than 30 genes unique to deuterostomes among animals, though some appear in marine algae and bacteria.
Fossils suggest the two branches split well before the Cambrian began 538.8 million years ago, since the 555-million-year-old Kimberella seems to be a protostome. By 521 million years ago echinoderms were common, and Haikouichthys and Myllokunmingia from China are the earliest fish body fossils. The shared ancestor was most likely a worm living on the seabed, with gill slits, a nerve cord and a skeleton of cartilage. Some recent genomic studies even question whether deuterostomes form one natural group at all.
Source: Deuterostome
