Life has found a house on Earth for around 4 billion years. That is a major fraction of the universe’s 13.77 billion-year history. Presumably, if life arose here, it could have appeared anywhere. And for sufficiently broad definitions of life, it’d even be possible for all times to have appeared mere seconds after the Big Bang.
To explore the origins of life, first we’ve to define it. There are over 200 published definitions of the term, which shows just how difficult this idea is to grapple with. For instance, are viruses alive? They replicate but need a bunch to accomplish that. What about prions, the pathogenic protein structures? Debates proceed to swirl over the road between life and nonlife. But for our purposes, we are able to use a particularly broad, but very useful definition: Life is every thing that is subject to Darwinian evolution.
This definition is handy because we’ll be exploring the origins of life itself, which, by definition, will blur the boundaries between life and nonlife. At one point, deep previously, Earth was not alive. Then it was. Because of this there was a transition period that can naturally stretch the bounds of any definition you may muster. Plus, as we dig deeper into the past and explore other potential options for all times, we wish to maintain our definition broad, especially as we explore the more extreme and exotic corners of the universe.
Related: Life can have evolved before Earth finished forming
With this definition in hand, life on Earth arose at the very least 3.7 billion years ago. By then, microscopic organisms had already turn into sophisticated enough to go away behind traces of their activities that persist to the current day. Those organisms were so much like modern ones: They used DNA to store information, RNA to transcribe that information into proteins, and the proteins to interact with the environment and make copies of the DNA. This three-way combo allows those batches of chemicals to experience Darwinian evolution.
But those microbes didn’t just fall out of the sky; they evolved from something. And if life is anything that evolves, then there needed to be a less complicated version of life appearing even earlier in Earth’s past. Some theories speculate that the primary self-replicating molecules, and hence the only possible type of life on Earth, could have arisen as soon because the oceans cooled, well over 4 billion years ago.
And Earth may not have been alone — Mars and Venus had similar conditions at the moment, so if life happened here, it can have happened there, too.
The primary life amongst the celebs
However the sun was not the primary star to ignite into fusion; it’s a product of a protracted line of previous generations of stars. Life as we understand it requires just a few key elements: hydrogen, oxygen, carbon, nitrogen and phosphorus. Apart from hydrogen, which appeared in the primary couple of minutes after the Big Bang, all of those elements are created within the hearts of stars during their life cycles. So, so long as you could have at the very least one or two generations of stars living and dying, and thereby spreading their elements out into the broader galaxy, you may have Earth-like life appearing within the universe.
This pushes the clock back on the possible first appearance of life to well over 13 billion years ago. This era within the history of the universe is often known as the cosmic dawn, when the primary stars formed. Astronomers aren’t exactly sure when this transformative epoch took place, however it was somewhere inside just a few hundred million years after the Big Bang. As soon as those stars appeared, they may have began creating the vital elements for all times.
So, life as we understand it — built on chains of carbon, using oxygen to move energy, and submersed in a shower of liquid water — could also be much, much older than Earth. Even other hypothesized types of life based on exotic biochemistries require an analogous mixture of elements. For instance, some alien life may use silicon as an alternative of carbon as a basic constructing block or use methane as an alternative of water as a solvent. Regardless of what, those elements have to return from somewhere, and that somewhere is within the cores of stars. Without stars, you may’t have chemical-based life.
The primary life within the universe
But perhaps it’s possible to have life without chemistry. It’s hard to assume what these creatures is perhaps like. But when we take our broad definition — that life is anything subject to evolution — then we do not need chemicals to make it occur. Sure, chemistry is a convenient approach to store information, extract energy and interact with the environment, but there are other hypothetical pathways.
For instance, 95% of the energy contents of the universe are unknown to physics, literally sitting outside the known elements. Scientists aren’t sure what these mysterious components of the universe, often known as dark matter and dark energy, are fabricated from.
Perhaps there are additional forces of nature that work only on dark matter and dark energy. Perhaps there are multiple “species” of dark matter — a whole “dark matter periodic table.” Who knows what interactions and what dark chemistry play out within the vast expanses between the celebs? Hypothetical “dark life” can have appeared within the extremely early universe, well before the emergence of the primary stars, powered and mediated by forces we don’t yet understand.
The chances can get even weirder. Some physicists have hypothesized that within the earliest moments of the Big Bang, the forces of nature were so extreme and so exotic that they may have supported the expansion of complex structures. For instance, these structures might have been cosmic strings, that are folds in space-time, anchored by magnetic monopoles. With sufficient complexity, these structures could have stored information. There would have been loads of energy to go around, and people structures could have self-replicated, enabling Darwinian evolution.
Any creatures existing in those conditions would have lived and died within the blink of a watch, their entire history lasting lower than a second — but to them, it might have been a lifetime.