Did life arise before Earth formed?


How did life arise on Earth?

It’s a good question and one that has intrigued people for thousands of years.

Every major religion has an origin story, but they’re not based on scientific evidence. Charles Darwin’s On the Origin of Species explored how the variations within species arose, but stopped short of talking about the origin of life itself.

Even now, over a hundred and fifty years later, we’re not much closer to understanding how life first arose as the evidence is scant. There is, however, an interesting theory that life may be older than Earth itself.

The oldest evidence for life on Earth is trapped in rocks over four billion years old.

Graphite deposits found in Zircon. Image credit: E A Bell et al, Proc. Natl. Acad. Sci. USA, 2015.

Tiny graphite deposits trapped in zircon diamond fragments reveal carbon-12 ratios that suggest life existed 4.1 billion years ago, which is astonishing given the planet itself is only 4.5 billion years old. How could life arise so quickly in the hostile environment of early Earth?

We’ve looked at the phylogenetic tree of life and assumed the point of origin from where all species emanated occurred shortly after the formation of the planet, but the genomic evidence suggests otherwise.

The phylogenetic tree of life shows the relationship between all species on Earth

The human genome contains roughly 23,000 genes, being built up from six billion nucleotide pairs, the biological alphabet of G, A, T & C (guanine, adenine, thymine and cytosine). As impressive as that sounds, the humble tomato has almost 32,000 genes, far more than we have. Over billions of years of evolution, DNA has grown to astonishing levels of complexity.

When scientists look at the complexity of non-redundant functional nucleotides, an interesting relationship emerges between genome size and the evolution of various organisms.

The complexity of organisms as measured by the length of functional non-redundant DNA in the genome increases with time. Image credit: Shirov & Gordon (2013), via https://arxiv.org/abs/1304.3381

On the surface, it’s not surprising to see genome complexity increase over time, but if we run the tape backwards, we find this relationship suggests that life is older than Earth itself, originating somewhere between nine and ten billion years ago.

There are some interesting implications to this finding, as discussed in a research paper published on Arxiv.

  • Earth was seeded by panspermia, where life arose elsewhere, was blown into space with the death of a star, and eventually arrived here on comets and asteroids
  • Life took a long time (almost 5 billion years or longer than Earth has been around) to reach the complexity of even simple bacteria like prokaryotes
  • The environments in which life originated and evolved to a pre-prokaryote stage may have been quite different from Earth
  • As the universe is 13.7 billion years old, carbon-based DNA life arose within 4 billion years of the Big Bang and may have spread remarkably wide
  • The slow progression of genomic complexity suggests there was no intelligent life in our universe prior to the origin of Earth, thus Earth could not have been deliberately seeded with life by intelligent aliens (Sorry Prometheus fans, no engineers)
  • The Drake equation for guesstimating the number of civilizations in the universe is likely wrong, as intelligent life would have only just begun appearing in the universe

Is it really such a surprise that life could have originated beyond Earth, billions of years before the planet formed?

Every element on Earth came into existence outside our solar system. The formation of our Sun simply dragged together the debris and detritus left over from a previous generation of stars.

All stars end their lives by throwing off their outer shells, and destroying their solar systems, suggesting a natural mechanism for panspermia (the seeding of life throughout the universe).

We’ve known for decades this process included inorganic material, like gold, silver and platinum, but now it seems it may have also included the frozen remnants of microbial life on other worlds.

If this theory is correct, then we may well find evidence for life on Mars, and perhaps living organisms on Europa and Enceladus, as they would have been seeded at the same point in time as Earth. Such life would be distantly related to us, having branched away from all terrestrial lifeforms before prokaryotic bacteria evolved on Earth. Also, it means other stars that formed from the same stellar nursery some five billion years ago, may harbor distant DNA cousins.

It’s a speculative theory, but based on an interesting observation, and accounts for the astonishingly quick rise of life on Earth. Regardless of whether it’s correct, the challenge we face when it comes to abiogenesis is in understanding how sub-prokaryotic life developed in the first place. The advantage of this theory is it gives life plenty of time to develop, which is actually more plausible than the rapid development necessary if life first arose on Earth.

In billions of years time, the Sun will blow off its outer shell and decimate Earth, and panspermia will begin again. Will other lifeforms arise from our celestial ruins? Will they develop intelligence and figure out the genomic path that lead to their formation, and perhaps gain a glimpse of life on this planet?

As much as I love writing science fiction, it is science itself that is often stranger and wilder than anything I can imagine.

 

 

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