Science Questions

Has life only started once on Earth?

Sat, 15th Sep 2012

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Question

Barrow Wheary asked:

If Earth is such a great place to live, why to our knowledge has life only begun once on this planet?

 

Obviously, the conditions on Earth are sufficient to support life. But beyond that, we repeatedly hear the assertion that these are the "ideal" conditions for life.

 

If this were so, why don't we see new life beginning here every day? And why do we believe that there is only one tree of life?

 

Is it possible that life began more than once or even continues emerging, say around deep ocean vents, but perhaps it occurs on a scale too microscopic or slow for us to readily observe?

 

Or is it possible that the conditions or events needed to "spark" life are quite different from those that simply sustain it? If so, are there any theories regarding the peculiar nature of such an environment or event?

Answer

We put this to Dr. Nick Lane, Lecturer in Genetics Evolution and Environment at the University College London. And started by asking him to define what life on Earth as we know it actually is...

Nick -   All life as we know it on Earth is composed of cells with an enclosing membrane, DNA as hereditary material, encoding proteins and so on.  All cells share a lot of their basic biochemistry right down to the same genetic code, the same ways of making proteins, even the same ways of generating energy.

DNA HelixHannah -   So, given that, has life as we know it or indeed alternative forms of life arisen more than once on Earth?

Nick -   We don’t know for sure that life only started once on Earth.  What we know is that all living organisms we’ve ever looked at are plainly related and so, almost certainly derived from a single common ancestor.  If life arose repeatedly then all of the separate origins must've disappeared without trace.  Perhaps out-competed to extinction very early on by the more successful cells.  The fossil record of bacterial cells goes back about 3,500 million years, that’s 3.5 billion years.  And so far as we can tell, these cells were very similar to their modern equivalents.

Hannah -   So, different forms of life may have started, but it seems that just our form of life survived over the last 3.5 billion years.  And where could our type of life have started?

Nick -   Deep ocean hydrothermal vents are the systems most likely to have given rise to life.  These far from equilibrium systems do at least favour the production of organic molecules, the biomass that makes up cells, but the level of self-organisation and dynamism required to get from cell matter to living cells is so great that we’re in no position to specify the probability of life arising even in the most favourable environment.

Hannah -   So, if we’re told that conditions on Earth are so suitable for forming life, then why aren’t new types of life arising all the time on present-day Earth?

Nick -   There are two big differences today which mean that life is almost certainly not continuing to arise even in these favourable settings.  One is the chemistry of the oceans.  Four billion years ago, there was no oxygen and much higher CO2 levels which together are much more conducive to life and then secondly, today’s vent systems are already packed with living cells which are far more effective at converting raw materials such as hydrogen and CO2 into biomass than any primitive prebiotic system could be.  So, however likely life might have been at the beginning, it’s extremely unlikely that life could originate again on Earth today.

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There are some theories that life began early on Earth, then went extinct during the Late Heavy Bombardment, or the Theia Impact Event, only to re-evolve a second time.

One thing that life does is that it shares molecules.  So, if an animal is eating a plant, then basic amino acids and DNA nucleotides can be re-used.  Thus, whether or not "life" evolved once, or multiple times on Earth, it might all look similar, and may tend to converge to a similar structure.

So, if one made a soup of extracellular DNA, RNA, amino acids, and fatty acids, and observed that life began.  Would this be a second evolutionary event?

While there are billions of different types of organisms on Earth, it may be difficult for a new life form to out-compete existing life. 

Consider a new bacterial form that could successfully divide once a year or so, without evolving any protection from normal bacteria, phages, and etc...  Unless well isolated, it might only last a few generations before being invaded and destroyed by the pre-existing organisms.

New DNA analysis techniques should help determine if there are any organisms that essentially do not share any genes with other organisms.  But, still, if the novel organisms had DNA, Fatty Acids, Proteins, and etc...  then likely they would share quite a bit with all other organisms. 

Perhaps one could say that we simply aren't looking for microscopic organisms that lack DNA.  But, we would have noticed any organisms that could be cultured that lacked DNA. CliffordK, Sun, 9th Sep 2012

There are bacteria absolutely everywhere on the Earth, and they'll be eating anything that might form by accident before it gets a chance to become anything. David Cooper, Mon, 17th Sep 2012

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