So does this imply that we'll never know if the Cambrian Explosion is actually an explosion or just a continuation from a previously unrecoverable geological record? Is it even possible to have complex multicellular life under snowball earth conditions?
>Although it’s likely down to a number of factors, one possibility is that Snowball Earth’s erosion was so significant that there wasn’t much topography left to erode when all was said and done. The planet simply needed to forge more land first, and that takes time.
The balance of earth is so interesting. The process that forges new land (volcanism) is the same process that beats back snowball earth and re-balances the atmosphere with more CO2.
> So does this imply that we'll never know if the Cambrian Explosion is actually an explosion or just a continuation from a previously unrecoverable geological record? Is it even possible to have complex multicellular life under snowball earth conditions?
That's a good question. I'm the first author of the new PNAS article discussed here (open access at https://doi.org/10.1073/pnas.1804350116). This does raise some questions about preservation bias and taphonomy, but there are areas we know escaped Neoproterozoic erosion (mostly at actively rifting continental margins) and we can find continuous sedimentary sections from the Cryogenian to the Cambrian -- and in these sections we still see a pretty rapid rise in complex shelly fossils in the early Cambrian. That said, when people first started talking about the "Cambrian Explosion", we didn't know much anything about the Ediacaran biota [1], the Ediacaran extension of the "Small Shelly Fauna" [2], or the Doushantuo Formation [3] -- so the evolutionary problem's not quite as bad as when Darwin first worried about it.
One new point to add is that erosion and comminution of this much crust may be expected to free up a lot of phosphorus stored in the igneous crust -- arguably the key limiting nutrient on geological timescales. This seems to be be consistent with previous observations of an increase in phosphorus abundance in sediments around Cryogenian times [4].
Oh, and regarding the survival of life through a Snowball Earth state, there's been some interesting discussion of potential refugia such as polynyas [1] and cryoconite ponds [2,3], though for context the most advanced multicellular organism I've seen anyone propose prior to the last snowball is still only a sort of sponge [4]
I find this stuff just absolutely fascinating, but have trouble finding reputable documentaries on the subject. YouTube is filled with "History" channel crap, and it's so disappointing to get all excited to learn, then be shown the crap they push out.
So, while apologizing for hijacking with an OT comment, do you have any docs you can recommend on the topic of geologic formation/periods of the Earth?
NOVA's always been my favorite science documentary series. I don't know if they've ever covered snowball earth, but they have a somewhat recent series on geological history of North America: https://www.pbs.org/wgbh/nova/series/making-north-america/
Likely there were a few 'explosive' events near the time of the Cambrian explosion. The Avalon Explosion [0,1] was about 33 million years earlier than the Cambrian Explosion. I would bet that our understanding of Life's history on Earth, and in the solar system in general, is only at the very beginning. Likely, our descendants will be looking at this understanding just like we look back at old 1890's paleontologists and their laughable Dinosaur models.
As an aside: I think that ML and RNNs would be a great tool for paleontologists to use for fossil hunting. Not just for automated drones scanning the lonelier places on Earth, but just to identify potentially 'rich' areas in the first place. Via petroleum studies, we know a fair bit about the Earth and the layers of sediment under it. Using that data, I think it may be possible to help narrow down the areas where good fossilization may have occurred and then send the armies of grad students out to those places, helping increase the percentages for finds.
> The process that forges new land (volcanism) is the same process that beats back snowball earth and re-balances the atmosphere with more CO2.
And I believe volcanism, at least the sort that is most common today (that from ocean-margin subduction zones, rather than that from spreading zones like Iceland or hotspots like Hawaii) results from subducted sediments melting into magma, with their melting points reduced by the presence of subducted water. If so, then do we have the large amounts of sediment, created by the snowball-earth glaciation, setting the stage for the volcanism that ends it?
There are fossils of Precambrian, post-snowball multi-cellular life, but they are not very much like, and nowhere near as diverse, as what appeared in the Cambrian, so I think the explosion still looks like a real event and not just an artifact of an imperfect record.
Since the erased part of the geological record starts 100’s of millions of years before the climate event that froze everything, do we even know that some earlier civilization didn’t force the climate off balance with e.g. greenhouse gasses, which ended up obliterating all multicellular organisms, and wiping the geological evidence?
Other recent news about ocean current disruptions, and methane melting feedback loops in the artic suggest we might be headed that way too...
And it would be written in the genes - models of evolution that work out timing from mutation rates and assays of genetic sophistication don't show any hidden episodes in the record.
>Although it’s likely down to a number of factors, one possibility is that Snowball Earth’s erosion was so significant that there wasn’t much topography left to erode when all was said and done. The planet simply needed to forge more land first, and that takes time.
The balance of earth is so interesting. The process that forges new land (volcanism) is the same process that beats back snowball earth and re-balances the atmosphere with more CO2.
It's a bit old and cheesy, some of the info might be outdated, but this is a great episode on this topic: https://www.youtube.com/watch?v=XEiu611KsUo