Why Hasn’t Life Appeared on Earth Multiple Times?
By: Dale Thomas
Tags: Life, Biology, Science, Evolution, Chemistry

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.