by Life has found a home on Earth for about 4 billion years. That’s a significant fraction of the universe’s 13.77 billion year history. Probably, if life came here, it could appear anywhere. And for a rather broad definition of life, it may even seem that life is only a second after the Big bang.
To investigate the origin of life, we must first define it. There are over 200 published definitions of the term, which shows how difficult this concept is to tackle. For example, are viruses alive? They replicate but need a host to do so. How about princes, the pathogenic protein structures? Debates continue to swirl about the line between life and heaven. But for our purposes, we can use a very broad, but very useful definition: Life is everything that is subject to Darwinian evolution.
This definition is convenient because we will be investigating the origin of life itself, which will, by definition, blur the boundaries between life and non-life. At one point, deep in the past, World was not alive. Then it was. This means that there was a transitional period which will naturally stretch the limits of any definition you can muster. Furthermore, as we dig deeper into the past and explore possible alternatives to life, we want to keep our definition broad, especially as we explore the more extreme and exotic corners of the globe.
Related: Life may have changed before the Earth finished forming
With this definition in hand, life on Earth arose at least 3.7 billion years ago. By then, microscopic organisms had already become sophisticated enough to leave behind traces of their activities that continue to this day. Those organisms were very similar to 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 from the sky; they arose from something. And if life is anything that evolves, a simpler version of life must have appeared even earlier in Earth’s past. Some theories speculate that the first self-expressing molecules, and thus the simplest possible form of life on Earth, could have emerged as soon as the oceans cooled, well over 4 billion years ago.
And maybe Earth wasn’t alone – Mars and Venus Conditions were similar then, so if life happened here, it might as well have happened there.
Download the first life among the stars
But the Sun the first star was not lit in fusion; he is the product of a long line of previous generations of stars. Life as we know it requires several key elements: hydrogen, oxygen, carbon, nitrogen and phosphorus. With the exception of hydrogen, which appeared in the first few minutes after the Big Bang, all these elements are created in the cores of stars during their lifetimes. So, as long as you have at least one or two generations of stars living and dying, thereby spreading their elements out into the world. galaxyyou can be Earth-like life seen in the universe.
This puts the clock back on the first possible appearance of life to over 13 billion years ago. This era in the the history of the universe called the Cosmic dawn, when the first stars were formed. Astronomers are not sure exactly when this transformative epoch occurred, but it was somewhere within a few hundred million years after the Big Bang. As soon as those stars were visible, they could have started creating the elements necessary for life.
Therefore, life as we know it – built on chains of carbon, using oxygen to carry energy, and submerged in a bath of liquid water – could be much older than Earth. Other hypothetical life forms based on exotic biochemistry require a similar combination of elements. For example, some alien life might use silicon instead of carbon as a basic building block or use methane instead of water as a solvent. Whatever it is, those elements have to come from somewhere, and that somewhere is in the heart of the stars. That starsyou cannot live a chemically based life.
The first life in the universe
But maybe you can have a life without chemistry. It is difficult to imagine what these creatures would be like. But if we take our broad definition – that life is subject to any evolution – then we don’t need chemicals to do it. Sure, chemistry is a convenient way to store information, extract energy and interact with the environment, but there are other hypothetical ways.
For example, 95% of the energy content of the universe is unknown to physics, literally sitting outside the known elements. Scientists are not sure what these mysterious components of the universe are called dark dark and dark energymade of.
Perhaps there are additional forces of nature that only work on dark matter and dark energy. Maybe there are multiple “species” of dark matter – a whole “periodic table of dark matter. ” Who knows what interactions and what dark chemistries play out in the vast expanses between the stars? A hypothetical “dark life” could have appeared in the universe very early, long before the emergence of the first stars, driven and mediated by forces we do not yet understand.
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The possibilities can get even weirder. Some physicists have hypothesized that the forces of nature were so extreme and exotic that they could support the growth of complex structures. For example, these structures could be cosmic strings, which is folded in space-time, anchored by magnetic monopoles. With sufficient complexity, these structures could store information. There would be a lot of energy to go around, and those structures could be self-replicating, enabling Darwinian evolution.
Any creature that existed in those conditions would have lived and died in the blink of an eye, their entire history lasting less than a second – but for them, it would be a lifetime.
