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If I understood correctly, all bilaterians come from a single organism (the Urbilaterian) that showed this type of body plan for the first time at the end of the Ediacaran.

Does that mean that bilateral symmetry evolved just once in the entire history of life? Is it correct to say then that bilateral symmetry is not a feature of convergent evolution? That this is indeed a thing that evolution explored just that time in history and never again?

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    $\begingroup$ It's currently hypothetical. pre-cambrian fossils are necessary from the 1bn-600 million region to find protostome - deuterostome ancestor. Genetic data is also inconclusive. some information if you search for bilateral common ancestor. i.e. dev.biologists.org/content/129/13/3021 indeed the common view is that there is a common ancestor. $\endgroup$
    – bandybabboon
    Mar 21, 2020 at 5:24
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    $\begingroup$ @com.prehensible Let me see if I got it right. One can then say that bilaterality is a feature that appeared only once in life's history and that this is what we think right now? Even if there is no enought evidence to flasify this idea yet and we wouldn't be very surprised to see that it happened twice or more times during the evolution of life. $\endgroup$
    – Swike
    Mar 21, 2020 at 19:13
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    $\begingroup$ Yes, all the baliteria are classed in the common clade which have a similar cell attributes and physiology, they all have hearts whereas starfish and jellyfish don't, there are various genes that suggest common ancestry. There is currently no evidence of bilateral from parallel evolution, although it's very possible or probable that there were some previously and perhaps even today, i.e. tardigrades don't have hearts, so perhaps some species one day will be found to be convergeant species. $\endgroup$
    – bandybabboon
    Mar 22, 2020 at 10:19
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    $\begingroup$ @com.prehensible I'm not a biologist at all so I would like your opinion on this. I've found that the Sea Cucumber has "weak bilateral symmetry" while been an Echinoderm (and thus a descendant of radially symetric creatures). Could this be considered bilaterality? If so, then we have some evidence for convergent evolution right? Source: en.wikipedia.org/wiki/Sea_cucumber#Body_plan $\endgroup$
    – Swike
    Mar 23, 2020 at 20:02
  • $\begingroup$ Sea cucumbers and sea anenomes both have a bilateral juvenile stage, so they have perhaps evolved from bilateral ancestors, check google scholar for "bilateral sea cucumber". researchgate.net/profile/Taha_Soliman4/publication/262415405/… A new fossil of a bilateral animal has been found from 550mn years ago: phys.org/news/2020-03-ancestor-animals-australian-fossils.html squids are starfish with mollusks on them, there are hybrid symmetries. $\endgroup$
    – bandybabboon
    Mar 24, 2020 at 7:16

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A lack of strong evidence causes the field to be mostly hypothetical. Detailed pre-cambrian fossils are necessary, from the 1bn-600 million years ago, to find a protostome - deuterostome ancestor.

Genetic data is also inconclusive. A search for bilateral common ancestor gives i.e. dev.biologists.org/content/129/13/3021 It's a summary of research, where the accepted view is that there is a common ancestor to all known bilateral organisms.

All the baliteria are classed in the common clade which have a similar cell attributes and physiology, they all have hearts whereas starfish and jellyfish don't, there are various genes that suggest common ancestry. There is currently no evidence of bilateral from parallel evolution, although it's very possible or probable that there were some previously and perhaps even today, i.e. tardigrades don't have hearts, so perhaps some species one day will be found to be convergeant species, although they are thought to be miniaturized lobopodia.

Sea cucumbers and sea anenomes both have a bilateral juvenile stage, so they have perhaps evolved from bilateral ancestors. researchgate.net/profile/Taha_Soliman4/publication/262415405/…

A new fossil of a bilateral animal has been found from 550mn years ago: phys.org/news/2020-03-ancestor-animals-australian-fossils.html squids are starfish with mollusks on them, there are hybrid symmetries.

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