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What was the first living thing on Earth?

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Abiogenesis: What was the first living thing on Earth?

We don’t know.

The difficulty is that there are no fossils of individual bacteria and viruses and the kinds of thing we presume were the first living things (perhaps an RNA molecule in a lipid bubble). Their chemical components would long ago have degraded - and even the rocks that are old enough to contain any fossil evidence have almost entirely been subducted and melted back into the Earth’s core.

Here is a photo of the very earliest evidence we have for life:

This is a bunch of tiny filaments, blobs and tubes in Canadian rocks dated to be up to 4.28 billion years old. They have the right chemical composition to represent a once-living thing - and the structure seems right.

But this wasn’t the VERY first living thing…it’s just the oldest one we have evidence for.

The FIRST living thing would have been a simple self-replicating molecule.  Something which, by virtue of very mundane chemical processes makes copies of itself from ingredients just floating around in the ocean (or wherever it originated).

The creation of that first ever self-replicating molecule is called the "abiogenesis event"...and science doesn't know much about how it happened.

It’s essentially impossible to ever find conclusive and direct evidence of the very first living thing, so we’re left with more of an experimental approach:

  1. Understand the nature of the environments available here on Earth over the range of a few hundred million years that we know life must have arisen.
  2. Try to figure out a chemical pathway from those conditions to some kind of self-replicating molecule…possibly RNA…possibly something yet simpler.
  3. Replicate those environments in the laboratory.
  4. Demonstrate the spontaneous creation of self-replicating molecules.
  5. Consider the evolutionary path from there to the earliest life that we have actual evidence of.
  6. Try to use this experimental work to make a testable prediction about early life…maybe some chemical that we might find in those oldest rocks that would be the “smoking gun”.
  7. Test the prediction by analysing a bunch of old rocks.

This is not a comfortable way to proceed - because all we’d ever be able to show is that spontaneous generation of life is possible via our best-guess route. We still couldn’t tell whether there might be other possible routes - and which of those routes actually happened.

Another problem is that of probability.

The approach above works well if whatever happens in step (4) is a high-probability thing…we set up the conditions in the lab - wait a month, then look for life and find it.

But if the probability of this spontaneous generation of life is very low - then we’ll never see it in a laboratory experiment - even if it runs over a square mile of early-earth environment and we let it run for a century.

Here on Earth, there were perhaps a hundred million square miles of ocean - and it could have taken 200 million years of random recombination of amino acids for just one suitable event to occur to create that first self-replicating molecule.

It could be even less likely than that. There are probably billions of suitable planets in the Milky Way and at least 100 billion similar galaxies in the observable universe. Life only had to appear once in all of those places (and that’s where we live). If the universe is truly infinite (which could easily be the case) - then no matter how statistically unlikely it is for life to arise spontaneously - then it would definitely happen…and again, that place was here.

If it’s that unlikely - then we may never see this happen in a laboratory.


In the “extremely improbable” case, we’ll never be able to show direct evidence for the creation of life.

We’d be left with exactly what we know now:

  • That the “ingredients” for life (things like amino acids and lipids) arise spontaneously in the conditions of early Earth. That experiment was done in 1952 (Miller–Urey experiment) - and it worked beautifully. Every one of the amino acids in DNA and RNA were produced from the chemicals we know were present in the early Earth - plus lightning bolts and/or volcanic activity.
  • Amino acids naturally form long chains.
  • A single self-replicating molecule did come together - presumably by random chance combination of those amino acids.
  • In replicating, however slowly and with however many errors, evolution would inevitably make it stronger and better.
  • Given that, it is obvious that early life must emerge.
  • And from that point onwards, evolution demonstrates that that primitive life would become the life we see in the earliest fossil evidence.
  • And from then, we know the entire story that leads to giraffes, humans, etc.

If we calculate the probability of that spontaneous creation of that relatively complex molecule - even with the most pessimistic assumptions…we know that the probability cannot be zero. If it isn’t zero then somewhere in all the oceans of all the planets of all of the stars in all of the galaxies and over a span of 14 billion years…life was inevitable - and we are the result.

So in a sense, we already know the entire story.

The trouble is, for the general public’s understanding, it’s an uncomfortable answer. It would be nice to show that the probability of this happening is high enough that life happens easily...but if it's not, then it's a harder argument still.

We’d absolutely want to be able to create life ‘spontaneously’ in the lab…but there are no guarantees that this is plausible.


Reader Forum: What was the first living thing on Earth?

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From: oursainsburys  Date: 2018-12-06 09:10:36  

From: jim  Date: 2018-04-12 09:15:17  

It seems to me the early Earth when life began was quite a bit different than what is currently assumed. For one thing the CO2 now residing in rocks was in the atmosprere and ocean earty on. And there was no land as we know it way back then. All the land we now have on our planet was magma in the beginning and slowly transformed to basalt to become land. As for the CO2 it is about 2% of the total mass of Earth's outer crust(depending how deep the crust is assumed to be) So all that CO2 being in the air&water would have made life impossible for quite a long time or until enough CO2 was absorbed by the magma transformation process