According to a recent study, it has become clear that the genetic circuit for segmentation “was inherited from a common segmented ancestor thought to have lived 600 million years ago and whose presence “changed the face of the world.”” According to the abstract of the study itself:
Annelids and arthropods share a similar segmented organization of the body whose evolutionary origin remains unclear. The Hedgehog signaling pathway, prominent in arthropod embryonic segment patterning, has not been shown to have a similar function outside arthropods. We show that the ligand Hedgehog, the receptor Patched, and the transcription factor Gli are all expressed in striped patterns before the morphological appearance of segments in the annelid Platynereis dumerilii. Treatments with small molecules antagonistic to Hedgehog signaling disrupt segment formation. Platynereis Hedgehog is not necessary to establish early segment patterns but is required to maintain them. The molecular similarity of segment patterning functions of the Hedgehog pathway in an annelid and in arthropods supports a common origin of segmentation in protostomes.
The researchers are now working on confirming the same circuit is behind vertebrate segmentation. Apart from this being another example of deep homology, where architecture and components shape and channel subsequent evolution for hundreds of millions of years, there is another feature to this story with telic echoes.
The ScienceDaily staff end their story with a tantalizing paragraph:
This work supports the idea that segmentation only appeared once in the history of evolution and that it led to the broad diversity of animal groups possessing it. This old and controversial idea among zoologists, had never been proved until now. But why should segmentation be so advantageous? Over millions of years, and exposure to changing environmental constraints, it is easier for an animal to specialize a segment into a specific tool in response to a need, than to create a whole new organ from scratch. By chance, evolution may have played a winning card with segmentation, which profoundly marked the history of life on Earth. If one day we could play God and create artificial animals or even biomimetic robots, perhaps we too should think about it. But this is still within the realm of science fiction.
Two themes emerge here.
First, Stephen Jay Gould’s Wonderful Life influenced the thinking of many scientists with his “replay the tape of life” metaphor. Gould argued the vast array of forms in Cambrian strata taught us that many more phyla existed in the Cambrian that exist today and that the subset of phyla which escaped extinction and exist to this day could have easily turned out to be a completely different set by chance. Since then, Simon Conway Morris has shown that Gould vastly overstated the diversity of the Cambrian strata and argued that most forms fall within known phyla or between them.
Yet Gould’s argument is further damaged by this finding. Since segmentation is so advantageous in terms of evolvability, it would still emerge and come to predominate even if Gould’s interpretations of Cambrian diversity had held up. Replay the tape of life from Gould’s notion of the starting point and hundreds of millions of years later, the segmented ancestor, with Hedgehog signaling pathway in place, would still have likely exerted its dominance.
And that brings us to the second theme: “But why should segmentation be so advantageous? Over millions of years, and exposure to changing environmental constraints, it is easier for an animal to specialize a segment into a specific tool in response to a need, than to create a whole new organ from scratch.”
This is the logic behind the success and dominance of gene duplication in evolution. In other words, here is another area where the logic of evolution itself emerges – the same strategy at the genetic/molecular level emerges to play a role in morphological evolution. Another conceptual connection.
One last point:
By chance, evolution may have played a winning card with segmentation, which profoundly marked the history of life on Earth.
But why think this is “by chance?” As I have argued earlier, the Hedgehog signaling pathway may have been front-loaded. So the Hedgehog signaling pathway facilitates the emergence of segmentation and segmentation, and borrowing from the logic of evolution-by-gene-duplication, confers a huge advantage to the lineage that expresses it. All of this profoundly marks the history of life on Earth. As planned.