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JMP: a statistical discovery software that makes  powerful analytics quick and accessible. You can head to JMP.com/scishow  for a free 30-day trial. One of the things I think is really  cool about studying paleontology is how much it’s like a detective story. Figuring out the past is often all about  clues, logical deductions, and timelines.

And, like any good detective  story, sometimes there’s a twist. A new piece of evidence  that flips the entire thing and forces us to ask what’s really going on here. And case-in-paleontological-point:  About 575 million years ago, there was a massive boom in animal  life, and for decades we’ve thought that the trigger was a whole bunch of extra  oxygen that appeared right beforehand.

But in today’s episode, we’ll explain  why that timeline might be backwards, and how paleontological  detectives pieced that together. [♪ INTRO] So it’s 575 million years ago and the Earth  has just thawed from a massive ice age. Down in the deep bottom of the oceans,  large, complex, multicellular life is first appearing in a big way. Life had existed on earth before this, but for the most part it had  been small, unicellular things.

Think, like, bacteria and algae, with the  occasional bigger splotch-of-a-critter. But suddenly the fossil record  just blooms with things like the feathery-looking rangeomorphs,  odd little disc-shaped fellows, and what may be early sponges or  coral-like cnidarians like Haootia. And some of these guys could  be big, almost two meters long.

This proliferation is called the Avalon Explosion. And while there would undoubtedly be even  larger and more spectacular life forms to come, you can think of this time period as kind of like Earth’s proof-of-concept  for ecosystems full of big, complex life. But as for what caused this bloom, that’s  where a bit of detective work comes in.

And one of the major clues for the cause of  all this is called the “Shuram excursion,” which kind of sounds like  the title of a mystery book. But rather than a bestseller,  the Shuram excursion was actually a massive, world-wide swing in  carbon isotopes recorded in rocks dating to right around the same  time as this Avalon Explosion. And one of the hypotheses to  explain the Shuram excursion is changes in the Earth’s carbon  cycle, which could have been brought on by the sudden appearance of a lot  of extra oxygen in the deep ocean.

The idea is that all that extra oxygen  would have chemically reacted with and released organic carbon stored in the  deep ocean, think dead algae and stuff, which would have then been spread  around the world and showed up in rocks, like those in the Shuram excursion. But scientists don’t know  exactly what would have caused this hypothetical increase in oxygen. If the Shuram Excursion was due  to extra oxygen in the deep ocean, it would seem to support the theory  that the mysterious explosion of life during that period was connected.

And Animals need oxygen to live, so if  there’s more oxygen in the deep ocean, you could have more animals down there, too. But, here’s the twist: a 2023  paper suggests that the isotopes, and by extension, the biodiversity bloom,  happened without any oxygen increase, which goes against what researchers  have thought for decades. And they came to this conclusion by going  back to the scene of the crime, so to speak.

They looked at the rocks in Oman,  called the Shuram Formation, that were the original evidence  for the Shuram excursion. They examined not just carbon, but also  two other elements, iron and thallium. Taking samples of the rocks, they used  X-ray fluorescence and mass spectrometry, which are, in short, ways to determine  what’s in a sample by bombarding it with X-rays and seeing how it glows or by comparing a particle’s  masses and charges, respectively.

And they concluded that, based on the  specific types of iron and thallium they found in the samples, that those rocks could  have only formed in oxygen-poor water. Not the oxygen-rich water that people  had assumed was around during that time. But… wait-wait-wait-wait… if it was oxygen poor, then that  means the excursion happened before any big oxygen increase… and if the animals  happened at the same time as the excursion then, that means they also came before.

It upsets the whole timeline. So that means, if this is right, that  multicellular life was able to develop just fine with way less oxygen than we  thought would have been required. It means that the conditions which first  gave the world multicellular organisms were way harsher and more demanding  of those early living things than we ever gave them credit for,  and that these creatures came about not because of the conditions of the  world they lived in, but in spite of them.

Now to be clear, this is  just one line of new evidence suggesting that oxygen isn’t the culprit. Other papers argue that there’s still plenty  of room for our friend O2 in this story. Which means the real answer to this  investigative puzzle is… we don’t know.

It’s still a mystery to be solved. Maybe this is a red herring, or  maybe this is the clue that’ll end up shining a new light on the whole case. But it does raise a lot of  questions about other assumptions.

Why would multicellular life  thrive in such harsh conditions? If oxygen wasn’t important, then what was? We don’t really have the  answers yet but rest assured, paleo-detectives are on the case.

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