Life might have been possible just seconds after the Big Bang

Life may be much, much older than Earth.

A composite image of the Bullet Cluster, a much-studied pair of galaxy clusters that have

collided head on. One has passed through the other, like a bullet traveling through an apple, and

is thought to show clear signs of dark matter (blue) separated from hot gases (pink). (Image

credit: X-ray: NASA/ CXC/ CfA/ M.Markevitch, Optical and lensing map: NASA/STScI,

Magellan/ U.Arizona/ D.Clowe, Lensing map: ESO/WFI)

Life has found a home on Earth for around 4 billion years. That's a significant

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

might even be possible for life to have appeared mere seconds after the Big Bang.

To explore the origins of life, first we have to define it. There are over 200

published definitions of the term, which shows just how difficult this concept is to

grapple with. For example, are viruses alive? They replicate but need a host to do

so. What about prions, the pathogenic protein structures? Debates continue to

swirl over the line between life and nonlife. But for our purposes, we can use an

extremely broad, but very useful definition: Life is everything that's 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 in the past, Earth was not alive. Then it was. This means that there was a

transition period that will naturally stretch the limits of any definition you can

muster. Plus, as we dig deeper into the past and explore other potential options

for life, we want to keep our definition broad, especially as we explore the more

extreme and exotic corners of the universe.

Related: Life may have evolved before 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 persist to the present day. Those organisms

were a lot 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 had to be a simpler version of life

appearing even earlier in Earth's past. Some theories speculate that the first self-

replicating molecules, and hence the simplest possible form of life on Earth, could

have arisen as soon as the oceans cooled, well over 4 billion years ago.

And Earth may not have been alone — Mars and Venus had similar conditions at

that time, so if life happened here, it may have happened there, too.

The first life among the stars

But the sun was not the first star to ignite into fusion; it is a product of a long line

of previous generations of stars. Life as we know it requires a few 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 of these

elements are created in the hearts of stars during their life cycles. So, as long as

you have at least one or two generations of stars living and dying, and thereby

spreading their elements out into the wider galaxy, you can have Earth-like life

appearing in the universe.

This pushes the clock back on the possible first appearance of life to well over 13

billion years ago. This era in the history of the universe is known as the cosmic

dawn, when the first stars formed. Astronomers aren't exactly sure when this

transformative epoch took place, but it was somewhere within a few hundred

million years after the Big Bang. As soon as those stars appeared, they could have

started creating the necessary elements for life.

So, life as we know it — built on chains of carbon, using oxygen to transport

energy, and submersed in a bath of liquid water — may be much, much older

than Earth. Even other hypothesized forms of life based on exotic biochemistries

require a similar mixture of elements. For example, some alien life may use silicon

instead of carbon as a basic building block or use methane instead of water as a

solvent. No matter what, those elements have to come from somewhere, and

that somewhere is in the cores of stars. Without stars, you can't have chemical-

based life.

The first life in the universe

But perhaps it's possible to have life without chemistry. It's hard to imagine what

these creatures might be like. But if we take our broad definition — that life is

anything subject to evolution — then we don't need chemicals to make it happen.

Sure, chemistry is a convenient way to store information, extract energy and

interact with the environment, but there are other hypothetical pathways.

For example, 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, known as dark matter and dark energy,

are made of.

Perhaps there are additional forces of nature that work only on dark matter and

dark energy. Maybe there are multiple "species" of dark matter — an entire "dark

matter periodic table." Who knows what interactions and what dark chemistry

play out in the vast expanses between the stars? Hypothetical "dark life" may

have appeared in the extremely early universe, well before the emergence of the

first stars, powered and mediated by forces we do not yet understand.

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The possibilities can get even weirder. Some physicists have hypothesized that in

the earliest moments of the Big Bang, the forces of nature were so extreme and

so exotic that they could have supported the growth of complex structures. For

example, these structures could have been cosmic strings, which are folds in

space-time, anchored by magnetic monopoles. With sufficient complexity, these

structures could have stored information. There would have been plenty of

energy to go around, and those structures could have self-replicated, enabling

Darwinian evolution.

Any creatures existing in those conditions would have lived and died in the blink

of an eye, their entire history lasting less than a second — but to them, it would

have been a lifetime.