Why Hasn’t Life Appeared on Earth Multiple Times?

Or, why abiogenesis can only happen once per planet

Photo by Maksym Ivashchenko on Unsplash

I hear this question quite often, especially from theists. People seem to think abiogenesis is purely a matter of luck and statistics. If it happened once, billions of years ago, then why hasn’t it happened in all that time since then?

This is a very cringe-worthy video of people not understanding the topic. It’s hard to refute this kind of thing because it’s just so fundamentally ignorant of the basics.

Life can’t suddenly appear in a jar of peanut butter. That’s not how it works; it’s not magic. It’s a process that requires a planet-wide ocean, full to the brim with self-organizing structures, amino acids, and autocatalytic chemicals, roiling and boiling, frothing and brothing, splishing, splashing and sloshing, for millions of years.

Before life existed, the oceans were a rich soup teeming with chemicals. There was nothing mopping them up. They remained free in the environment, interacting with each other, diffusing, spreading, mixing, dissolving, recrystallizing, evaporating, condensing, precipitating, etc. Once life had begun, however, it replicated and spread, sucking up all those free chemicals. Life drained the soup of flavor, leaving just the salty brine. All those chemicals are now locked up inside extant organisms, all part of the food chain.

There are still pockets of chemical and energy-rich environments, such as hydrothermal vents in the bottom of the ocean, that spew free chemicals out in boiling plumes, but they are surrounded by all kinds of complex life forms, consuming whatever free nutrients they can get their slimy tendrils on. They have had billions of years to evolve to become experts at extracting nutrients and are very good at it.

Life has no chance to form again because there is no free substrate for it to form on. The circumstances needed for it are no longer available, and will never be, as long as there is a single organism capable of reproduction.

When people think of the origin of life, they think of a single cell popping into existence. A lone amoeba floating in a vast ocean, the ancestor of life, the root of the tree. And if we open a cellular biology textbook and look at the inner workings of a cell, we see they are unfathomably complex. So the spontaneous emergence of one of those cells must be statistically impossible, right? Given that view of things, it’s intuitive to ask the question, how could that cell, with all its intricate molecular mechanisms, possibly come about by chance? It’s inconceivably improbable.

But the first form of life was nothing like modern-day cells. Today’s organisms have literally been evolving for nearly four billion years. Even amoeba. Their mechanisms have been honed and optimized to near-perfect through countless cycles of growth, division, and natural selection. The first ‘lifeform’ was not a cell as we imagine them today. It was a highly diffuse network of free chemicals that had the quality of being self-sustaining. There was no sophisticated cell wall or complex protein-folding machinery or error checking mechanisms or DNA scaffolding. Cells came later, as lipids began to cluster and self-organize into bubbles and sheets, trapping local reactions together, separating parts of the process.

The Miller-Urey experiment from the 50s showed that early earth conditions were favorable for the spontaneous formation of organic compounds, such as the amino acids that are the building blocks of life. While our understanding of the details of what those conditions were like has changed, the results seem to be quite robust against tweaks in the model.

Obviously, scientists will never know exactly how all this happened, as it’s not like there is any direct evidence left from that time. But there are a lot of intriguing theories, sophisticated simulations and models, and indirect evidence. So it’s not as if it’s just a guess.

Now, there is speculation among some scientists that life did emerge multiple times near the beginning.

The idea is that in the early days, with all the chemicals sloshing around over millions of years, local pockets of what could be called life emerged, and mixed with each other and battled for supremacy until there was only one ‘system’, the LUCA (Last Universal Common Ancestor):

The earth is big. It’s not as if once a life system appears that it instantly spreads across the world, blocking out the formation of any other local life system. Such a spread would not be quick, so it's possible that proto-life systems did emerge more than once. However, I still see that as life on earth emerging once. I think diffuse chemical networks mixing together is the formation of life. So, in my view, it’s all just splitting hairs. What we can say for sure, is that a complex cell didn’t just appear from nowhere. It wasn’t like there were all these ‘non-living’ atoms floating around and bumped into each other to become ‘living’ atoms, which is the strawman that is usually argued against.

Here is another view that says life emerged just once:

Again, I don’t really see it as an all-or-nothing question. Since we still don’t have a definite definition of life, it’s even harder to say which of these proto-life systems would count as abiogenesis in their own right, even if we could study them, which unfortunately we can’t.

Abiogenesis is one of the great questions of humanity that I believe will never have a definitive answer. It’s too complex and we don’t have the ability to rigorously study such conditions. Not really. And the reason it will never happen again is simply that the conditions will never be that same.

In an 1871 letter to botanist Joseph Hokker, Charles Darwin wrote the following regarding the idea of abiogenesis occurring again:

“It is often said that all the conditions for the first production of a living organism are present, which could ever have been present. But if (and Oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.”

When I think about the first lifeform on Earth, I don’t imagine a lone microscopic cell with big potential blinking into existence. I picture a planetwide ocean, with local pockets of complex chemical networks and abundant naturally occurring amino acids being smeared together by tides and currents, emerging slowly as a system capable of self-reproduction, gradually refining into cells over millions of years.

And once that life took hold, there was no way new life could get a look in.

Scifi writer, roboticist, and game developer, 2x Quora Top Writer. I write about writing speculative fiction, computer graphics, AI, evolution, and programming.

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