Origin of Body Plans
The origin of animal body plans: a view from fossil evidence and the regulatory genome
Douglas H. Erwin
Development 15 February 2020; 147 (4): dev182899.
An overview paper, rather than discussing research, but still very informative. Erwin touches on a lot of things that Mills and Canfield discuss in Oxygen and animal evolution but with a slightly different emphasis.
Erwin’s whole point is that changes in regulatory genes drove metazoan evolution by allowing greater number of cell types, or by letting animals do different body plans. He does this by examining common genes between phyla, and trying to correlate to the fossil record.
Erwin writes a bit about the “protostome deuterostome ancestor”, which has to be somewhat like the ur bilaterian. Erwin comes down on a very simple creature with left/right, anterior/posterior and dorsal/ventral “patterning”, but not complex morphology. The simple creature had quite a few cell types, but wasn’t segmented, didn’t have appendages, eyes, gut formation or segmentation. Nevertheless Erwin’s PDA is simpler than other folks’ takes on an ur bilaterian.
Erwin solves the problems of reconciling the possibilities by believing that segmentation and other characteristics evolved independently several times. He has vertebrates (deuterostomes), annelids (protostomes and lophotrochozoans), and arthropods (protostomes and ecdysozoans) all evolving segmentation. This is a lot like Spiculogenesis and biomineralization in early sponge animals proposing that the last common sponge ancestor did not have spicules, and various sponge groups evolved spicules independently.
The really interesting part of this paper is this phylogeny:
The solid bars indicate when fossils of a particular clade exist.
Divergence times are from genetic evidence had have long red error bars.
Nevertheless, the last common metazoan (node labeled 2)
might have existed before the Sturtian Snowball Earth.
That would make the
Snowball Earth driving multicellularity
paper moot - animals were possibly already multicellular.
They used to draw this kind of diagram based on the
traditional list of animal phyla.
Those diagrams didn’t make a lot of sense,
because they also used characteristics like segmentation
to try to group phyla,
so they had things annelids and arthropods as sister phyla.
This cladistic approach,
along with allow independent evolution of segmentation,
ends up making more sense.
Douglas H. Erwin
The origin of animal body plans: a view from fossil evidence and the regulatory genome.
Development 15 February 2020; 147 (4): dev182899.
doi: https://doi.org/10.1242/dev.182899