In recent years, the search for the origins of life is becoming complex. It turns out that oxygen is NOT a requirement for multicellular life to thrive. As stated in this BBC article, poriferans (sponges) can thrive at oxygen levels of just 0.5%. And according to Kitsap Sun, jellyfish do better in low-oxygen waters than fish. And then, of course, there are the loriciferans, a phylum whose common name I can't find. They, too, thrive with little to no need for oxygen.

So this got me thinking. Life first appeared 4.28 billion years ago as single-celled, extremophilic microbes, possibly bacteria or archaeans or both, thriving in a perfectly anaerobic Earth. But a recent discovery has shown that the jump from single-celled to multicellular is not as hard as traditionally assumed. With that in mind, could animals evolve sometime during the Hadean Eon, 4.6-4 billion years ago? Or would something hold them back?

• Life may, and its a big may, have begun in the hadean but if it it did it did so right at the end of the hadean, And it would have taken quite a bit of time for life to become complex enough a distinction between unicellular and multicellular could even be made. It has to evolve controlled division and inter cellular binding first. – John Oct 10 '18 at 4:24
• "jellyfish do better in low-oxygen waters than fish" Jellyish and Sponges still need to live in water though and Water wasn't around in any quatity on Earth in the Hadean Eon...hmm, Water, H2O... the O does stand for something... I just can't quite remember what... – Blade Wraith Oct 10 '18 at 6:59
• Animals are eukaryotes and most sources put the origin of eukaryotic cells at 2–2.1 billion years ago. So the Hadean is definitely out. – AlexP Oct 10 '18 at 10:38
• the article (have to read it more closely later) seems to be doing a lot of hind-sighting. per AlexP, a more compelling argument would be could eukaryotes have elaborated earlier. the simple, but powerful argument, is that the experience of ~1.5Ba of trials indicates it evidently required that time to accrete modifications to cell structure sufficient to scaffold persisting multi-cellular organisms, involving collagen for binding, elaboration of membrane & signalling capabilities, etc. just looking at (repurposed) genes doesnt show the critical path. – theRiley Oct 10 '18 at 11:43
• Some multicellular organisms do well in LOW oxygen, but not WITHOUT oxygen (as was pretty much the case during Hadean era). – Alexander Oct 10 '18 at 17:22

Or would something hold them back?

The environment held even bacterial life back.

Life is believed to have started in the archean, not in the hadean. According to this article from the Univerdity of California:

The Archean eon, which preceded the Proterozoic eon, spanned about 1.5 billion years and is subdivided into four eras: the Neoarchean (2.8 to 2.5 billion years ago), Mesoarchean (3.2 to 2.8 billion years ago), Paleoarchean (3.6 to 3.2 billion years ago), and Eoarchean (4 to 3.6 billion years ago). (...) Also during this time, the Earth's crust cooled enough that rocks and continental plates began to form.

It was early in the Archean that life first appeared on Earth. Our oldest fossils date to roughly 3.5 billion years ago, and consist of bacteria microfossils.

Hadean time (4.6 to 4 billion years ago) is not a geological period as such. No rocks on the Earth are this old, except for meteorites.

And from the wiki:

Liquid water oceans existed despite the surface temperature of 230 °C (446 °F) because at an atmospheric pressure of above 27 atmospheres, caused by the heavy CO2 atmosphere, water is still liquid. As cooling continued, subduction and dissolving in ocean water removed most CO2 from the atmosphere but levels oscillated wildly as new surface and mantle cycles appeared.

As if the a pressure cooker atmosphere wasn't enough, gas levels oscillated. I am quite sure no extremophile is that extreme. I've heard of bacteria withstanding close to 100 ºC, but 230 ºC, while also bearing 27 atm's, is pushing it.

But hey, it's your world, you can handwave that — and the coolest part is that it would not be unrealistic :) just because we haven't seen it doesn't mean there couldn't be such microbes. And if they thrive, then more complex lifeforms can evolve from them. It would just probably be very different from anything we've ever seen, both anatomically and physiologically.

• So could the Archaean be favorable for animal evolution, then? – JohnWDailey Oct 10 '18 at 2:21
• @JohnWDailey that is not probable, but not impossible either. – Renan Oct 10 '18 at 2:51
• Pressure and temperature at hydrothermal vents (en.wikipedia.org/wiki/Hydrothermal_vent) are even higher and bacteria do exist there too, forming the root of an entire food chain. The same article also mentions bacterial microfossils as early as 4.28 billion years, a mere 120 millions years after the formation of the liquid oceans. – GretchenV Oct 10 '18 at 8:06

## Not all single-celled organisms are equal

It's easy to take examples from modern times and try an extrapolate backwards, but later single-celled organisms have evolved a lot from the first living things. Even if you could come up with a hypothetical extremophile that could live in Hadean conditions, that doesn't mean life could have started under those conditions.

Archaea, contrary to what their name may suggest, are not like the first organisms at all - they are highly adapted cellular tanks with a ton of adaptations for surviving the inhospitable locations they are known to inhabit, and they evolved these abilities in order to exploit resources that other organisms could not. It is highly unlikely that life could have originated under these conditions.

Similarly, although the jump from single-cellular to multicellular life happened pretty fast, life was not just twiddling its thumbs for that first 3 billion years - it was evolving new chemical pathways, better DNA replication mechanisms, and the ability to interact with other cells. The framework upon which all multicellular life is founded is extremely complex and it took a lot of time to develop this framework; the first primitive self-replicating blobs were nowhere near advanced enough to develop into an animal.

Short Version : Not Animals, not plants.

could animals evolve sometime during the Hadean Eon, 4.6-4 billion years ago? Or would something hold them back?

We have only one example of the evolution of life to go on, so absolute statements are hard to make.

However ...

The first life is thought to have appeared in the Archean Era and the most direct evidence for that is for Stromatolites about 3.5 billion years ago, which are due to single celled microbes.

To get to animals, which is what you ask about, took until the Cambrian Era which was "a mere" 540 million years ago.

So based on our one example of evolution, it takes about 3 billion years from life to go from single cells to animals (and we're including very basic sea creatures in this loose definition of animal).

So if you start your evolutionary cycle in a Hadean Era, it's only about half a billion years before we think the first life appeared on Earth. So you shave about half a billion years off the time when animals appear.

Or would something hold them back?

This is an opinion, rather than something I can rigorously support, but I would suggest that while life based on the chemistry possible in the Hadean Era may be possible (I can't say it isn't does not mean I think it is), life based on the more stable geological conditions and different chemistry of the later Archean period may simply be better able to flourish.

Also the change of conditions might simply make it impossible for the multicelluar organisms that would (optimistically) be in existence at the end of the Hadean Era to continue. It's like any life existing in the Hadean would simply not be compatible with the conditions that developed later.

During the Hadean Era surface temperature is thought to be about $$230^\text{o}\,C$$ and atmospheric pressure was of the order of 27 atmospheres. By the Archean Era conditions have changed to (again very rough estimates) less than $$50^\text{o}\,C$$. As that era developed temperatures fell.

The issue here (and it woudl apply to any planetary formation) is that at the start conditions change (relatively) quickly. This will mean that anything adapted to the relatively extreme conditions of the earliest period will almost certainly not be compatible with later period.

Finally there is evidence of a period called the Late Heavy Bombardment which would have occurred at about the late Hadean-Early Archean period. This may or may not have affected the development of life on Earth, so it's a potential source of "wiping the slate clean". However I'd suggest (opinion) that what little evidence there is does not seem to support a break in the chain of evolution like this. It seems more likely that Hadean life forms (cells) would have evolved or died out as conditions changed due to the way the Earth's general state changed.

• There is doubt as to whether or not the LHB actually happened. – JohnWDailey Oct 10 '18 at 21:41

The issue with life in the Hadean isn't so much the oxygen, it's the water; the surface of the earth is too hot for any bodies, or even puddles, of liquid water to form. So life on the surface of the planet not so much, there are at least two modern lifeforms that might be able to survive "on" (in the atmosphere of) a planet in that state:

• Radiodurans, this little extremophile that can survive extreme cold, dyhydration, ionizing radiation, strong acids, and hard vacuum; they first found it floating on the edge of space, above the Kármán line and thriving in UV strong enough to sterilise surgical equipment. It could inhabit that same, insanely hostile, environment during the Hadean.

• any of the cloud borne or stratospheric bioaerosol community, I was thinking in particular of free floating mitochondrion but they would require free oxygen so probably not, there are similar creatures that exploit sulfur-oxide gases, methane and CO2 which might be candidates.