Protostomes and deuterostomes are two groups of animals that split over 558 million years ago. Protostomes include arthropods, molluscs, and flatworms, while deuterostomes include chordates, hemicordates, and echinoderms. The main difference between the two groups is that in protostomes, the mouth forms earlier in the embryo, and they exhibit spiral cleavage, while in deuterostomes, the anus forms first, and they exhibit radial cleavage. Protostomes are more abundant and simpler than deuterostomes, but deuterostomes comprise all complex animals.
Protostomes (Greek: mouth first) are a huge clade of animals that includes the Ecdysozoa (arthropods, nematodes and friends), the Platyzoa (flatworms and allies) and the Lophotrochozoa (molluscs, annelids and allies). Their counterparts, the deuterostomes, include the chordates, hemicordates, and echinoderms. There are several differences between the two groups, which all constitute bilateral animals, but the most frequently mentioned is that the mouth forms earlier in the embryo, rather than in the anus. This is a pretty fundamental difference.
Another difference between protostomes and deuterostomes is that in protostomes, the embryo exhibits spiral cleavage, where the position of each cell is deterministic. In deuterostomes, which show radial cleavage, there is less certainty about where the differentiated cells end up. The embryological development of protostomes could therefore be considered relatively conservative and predictable. Unsurprisingly, protostomes enclose the world’s simplest animals, including the first bilateral animals to appear in the fossil record, such as arthropods and molluscs.
When it comes to counting species, protostomes trump deuterostomes. While there are many millions of protostome species, especially thanks to the phylum Arthropoda, there are only about 70,000 deuterostome species, although these comprise all fairly complex animals. Protostomes comprise all simple animals, but their abundance is a testament to their success. Ignoring fish and whales, protostomes make up most of the biomass in the world’s oceans, although they are less dominant on land, where the vertebrate body plan has proven highly successful. The suspension qualities of water make the spine much less necessary.
Protostomes and deuterostomes are thought to have split at least 558 million years ago, and possibly much earlier. This is inferred from the existence of Kimberella, an early mollusk, 555 million years ago, which is a known protostome. However, just because Kimberella existed at the time, why does that mean deuterostomes didn’t evolve later, branching off from protostomes? There is little solid consensus on the matter, but apparently some Late Ediacaran (older than 542 million years) fossils, Ernettia, have a probable affinity to deuterostomes, although the consensus on the classification of Ediacaran fossils is often absent. In any case, it is certain that protostomes and deuterostomes separated at the dawn of the Cambrian, 542 million years ago.
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