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Prior to the Cambrian, there have been three separate ice ages--the Huronian, from 2.4 to 2.1 billion years ago; and the two Cryogenian ice ages, from 720 to 635 million years ago, split in such by a ten-million-year lull.

Post-Huronian came the first eukaryotes, cells with a nucleus concealed within membranes. The first of some of the multicellular eukaryotes, the animals, first appeared 665 million years ago, near the last legs of the Cryogenian... or did they? I ask that because one of the evidence questioning the occurrence of the Cambrian "Explosion" is the fact that trilobites, the most iconic organisms of the Palaeozoic Era, were already diverse by the time it allegedly happened, which would indicate that they evolved much earlier, at least 700 million years ago.

The fact that eukaryotes evolved in the wake of the Huronian was no problem. As this BBC site may indicate, isolation by ice may force the cells to be more specialized. But what does not make sense for me was that melting such a global snowball, as the Huronian essentially was, meant an increase not just of nutrients dumped into the oceans, but also free oxygen rising into the atmosphere, yet animals did not come into the stage until the Late Cryogenian, in which the melting did the exact same thing the melting of the Huronian did.

So my question was - Could multicellular life evolve with just the Huronian glaciations and not the two Cryogenian glaciations?

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    $\begingroup$ Pick one between hard science and science based, they are mutually exclusive $\endgroup$ – L.Dutch - Reinstate Monica Jan 11 at 3:39
  • $\begingroup$ @L.Dutch May I ask how? $\endgroup$ – JohnWDailey Jan 11 at 3:50
  • $\begingroup$ If you want scientific citations go for hard science, else go for science based. $\endgroup$ – L.Dutch - Reinstate Monica Jan 11 at 3:51
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    $\begingroup$ I am glad you are back, Dailey. I like your questions! $\endgroup$ – Willk Jan 11 at 6:12
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The earliest Metazoa originate between 800mya and 750mya, so anywhere from 80my to 30my prior to the beginning of the Cryogenian.

The occurrence of snowball periods is likely not a factor in their appearance. In fact, it has been suggested that the Cryogenian caused a loss of diverity in the Earth's oceans. This loss was not recovered until the glaciation ended, and the evolution of the Ediacaran fauna occurred.

My conclusion, therefore, is that the Cryogenian was not required for the evolution of animal life on Earth, and may have in fact inhibited it.

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  • $\begingroup$ So without the Cryogenian, something along the lines of the Great Ordovician Biodiversification Event could still happen, albeit a lot earlier? $\endgroup$ – JohnWDailey Jan 11 at 3:30
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    $\begingroup$ Entirely possible, yes. $\endgroup$ – Arkenstein XII Jan 11 at 3:40
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    $\begingroup$ you will not have arthropods, chordates, and mollusks but you will have your own equally complex things. There is a strong element of , for lack of a better word, "Luck" in getting the exact combination of features each of these groups has, if you rerun evolution will not get these exact same groups but you will get equally complex groups. $\endgroup$ – John Jan 11 at 5:01
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    $\begingroup$ @JohnWDailey What John is saying, is that there are so many different, and reasonably random, selection pressures acting on any given species, that the particular biological solutions that occurred in our history are not necessarily going to happen again. There will be different solutions that arise the the same pressures, and there will be different pressures that were not present in our timeline. There's a large degree of randomness inherent in evolutionary adaptation. $\endgroup$ – Arkenstein XII Jan 13 at 7:54
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    $\begingroup$ Arkenstein s correct, two of the strongest influences on evolution are what mutations arise (which is more or less random) and what other organisms in the environment are doing, which again is not predictable in the long term. It creates a feedback loop of contingency that makes exact prediction impossible. $\endgroup$ – John Jan 14 at 11:19
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It is proposed that the Precambrian ice ages stalled evolution because the cold stalled photosynthesis. This is demonstrably not so.

On land, photosynthesis by algae proceeds fine in the snow.

snow algae https://www.naturepl.com/stock-photo-green-snow-algae-petermann-island-antarctic-penninsula-antarctica-image01021676.html

In sea ice, photosynthesis also proceeds fine in subzero temperatures.

sea ice algae https://askabiologist.asu.edu/explore/frozen-life

During the Precambrian ice ages, sunlight would still be harvested and still be used to make oxygen. Evolving animal life would have access to the oxygen needed by multicellular life as we know it.

The animals were there after the Huronian. Trilobite ancestors for one, but also strange ones, the likes of which are hinted at in the Edicaran fossils. Probably there were predators too,, capitalizing on the resources stored by their sessile cousins,

But we do not know their shapes. A billion years elapsed between; time enough for the first stirrings of a maiden Earth to be lost. The thawing of the Cryogenian is the dawn of life not because life did not exist before, but time and change has obscured its traces in the darkness before dawn.

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According to this link, http://www.astrobio.net/origin-and-evolution-of-life/multicellular-life-evolve/amp/, multicellular life first appeared 3.5 billion years ago, about a billion years after the Earth was formed.

Multi cellular animals began to appear in the fossil record about 600 million years ago, right in the middle of the cryogenian. As you noted, they are already very diverse, confirming what we already know, that the fossil record is incomplete.

As you note, the diversity of animal life in the mid cryogenian period suggests that multicellular animal life may have been abundant pre-cryogenian.

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