The rise of free atmospheric oxygen and oceanic dissolved oxygen levels were almost certainly a requirement for the development of complex multicellular life. Using oxygen as the electron acceptor in respiration means far more energy can be extracted from reduced fuel molecules (sugars, fats, amino acids, methane, hydrocarbons, ammonia, etc.) than by using species like oxidized sulfur and iron, CO2, or nitrate (NO3) for that role.

This is really where the mitochondria come into play, as they allowed their host to utilize oxygen for this purpose. This can be seen by looking at modern eukaryotes that have reverted back to an anaerobic lifestyle:

> "In lineages of eukaryotes adapted to low oxygen conditions, mitochondria have been drastically reduced, functionally altered and, in one case, completely lost."

"The Origin and Diversification of Mitochondria (2017)"

https://www.cell.com/current-biology/pdf/S0960-9822(17)31179...

Hence, looking for the signature of free oxygen in the atmosphere of exoplanets orbiting distant stars is considered to be a fairly good indicator of the possibility of complex multicellular life of some sort, and at least of an active photosynthetic microbial ecosystem.

(Incidentally, the historical divisions withing academic university departments led to evolutionary biology generally ignoring the importance of early Earth's geochemistry in the evolution of life, as they saw evolution as a kind of cellular/organismal process divorced from the physical surroundings - the latter being the province of the geology department. The renewed interest in exobiology and origin-of-life research has tended to bridge this gap.)

The way I read that is that developing complex life is even more contingent than 'just wait long enough'. Given what we know for sure, you need rather narrow concentrations of specific gasses at least.

To your last parenthetical, organicism a la Hegel mends the blind spots once again!

This is Nick Lane's argument - that complex cell happend only once on Earth (no convergent evolution) via this symbiotic accident and hence it's highly improbable if not almost impossible.

As usual, people always seem to foreget that in this universe, what happened once, happens all the time and we are not special.

Nick Lane's book The Vital Question is a masterpiece. I reckon he's figured out the origin of life there. I hope he gets on some podcast like Lex Fridman to talk about it all, because his ideas are just waiting to explode into the public imagination.

He was just on Sean Carroll's podcast Mindscape, good listen: https://www.youtube.com/watch?v=9IZ_CVu2M68

Awesome! Thanks for the link, I'll give this a watch!

We're all just speculating given that were stuck at sample size = 1 until we develop near light speed travel.

I get the convergent evolution point, but so far the story is that 1) life arose very quickly 2) then proceeded to do very little for billions of years. If that's true, then it's bad news for the possibility of complex life being common.

> 1) life arose very quickly 2) then proceeded to do very little for billions of years.

This is understating the feats of unicelular life. Those tiny organisms essentially terraformed the whole planet in those billions of years, oxidizing nearly all the iron close to earth surface, burying most carbon and then pumping so much oxygen in the atmosphere that at a point it comprised 30% of Earth gaseous envelope by weight. I wouldn't call that very little. To come close we'll need at least to terraform Mars or Venus, to match the feats of our microbial forebears.

I think the parent comment meant doing very little from an evolutionary perspective.

It only means that complex life is less frequent then single cell life, not that it is uncommon.

> that complex cell happend only once on Earth

Well, it's quite possible that once it happens, it voids any chance of it happening again just because life filling that niche already exists. If so, it only happens once, and this says absolutely nothing about the odds of that first occurrence.

> what happened once, happens all the time

That's a fallacy people do all the time here when talking about exobiology. It's basically states that all small numbers and all big numbers are alike, and that all small numbers are like the inverse of all big numbers.

Most other features evolved independently several (or even many) times so this is not a good argument. It’s not as if once there was a eukaryote it instantly spread across the whole planet, taking up every ecological niche. Even today eukaryotes don’t exist in certain environments.

While no single eukaryote did what you said, eukaryotes as a whole did, to some extent, during the Cambrian Explosion

And that is if there is only one universe, which might not be so special as well.

Single-celled life is complex. It has the same length of evolutionary time as multicellular life.

We know very little about modes of sentience with N=1. Perhaps life is generally mutualistic.

It’s not wrong, at least I don’t think this article is arguing that multi cellular life evolved any earlier than we thought. Just that the mechanisms were different than we expected.

Nothing in the article contradicts that 'fact'. The question is only whether the development of eukaryotes involved other significant steps in addition to the mitochondrial endosymbiotic event, or whether the latter event was all that really mattered. In both cases, subsequent development of eukaryotes and hence multicellularity depended on the seemingly very rare endosymbiotic event.