[If life emerged once, why not more times? ]
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SUNDAY SCIENCE: DID LIFE EVOLVE MORE THAN ONCE? RESEARCHERS ARE
CLOSING IN ON AN ANSWER  
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Jordi Paps
May 18, 2023
The Conversation
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_ If life emerged once, why not more times? _

, Maximillian cabinet/Shutterstock

 

From its humble origin(s), life has infected the entire planet with
endless beautiful forms. The genesis of life is the oldest biological
event, so old that no clear evidence was left behind other than the
existence of life itself. This leaves many questions open, and one of
the most tantalising is how many times life magically emerged from
non-living elements.

Has all of life on Earth evolved only once, or are different living
beings cut from different cloths? The question of how difficult it is
for life to emerge is interesting – not least because it can shed
some light on the likelihood of finding life on other planets.

The origin of life is a central question in modern biology, and
probably the hardest to study. This event took place four billion
years ago
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and it happened at a molecular level – meaning little fossil
evidence remains.

Many lively beginnings have been suggested, from unsavoury primordial
soups to outer space. But the current scientific consensus is that
life emerged from non-living molecules in a natural process called
abiogenesis, most likely in the darkness of deep-sea hydrothermal
vents
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But if life emerged once, why not more times?

What is abiogenesis?

Scientists have proposed various consecutive steps for abiogenesis. We
know that Earth was rich in several chemicals, such as amino acids, a
type of molecules called nucleotides or sugars, which are the building
blocks of life. Laboratory experiments, such as the iconic Miller-Urey
experiment [[link removed]], have
shown how these compounds can be naturally formed under conditions
similar to early Earth. Some of these compounds could also have come
to Earth riding meteorites.

[Smoking hydrothermal vent.]

Smoking hydrothermal vent. NOAA/wikipedia.

Next, these simple molecules combined to form more complex ones, such
as fats, proteins or nucleic acids. Importantly, nucleic acids —
such as double-stranded DNA or its single-stranded cousin RNA
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store the information needed to build other molecules. DNA is more
stable than RNA, but in contrast, RNA can be part of chemical
reactions in which a compound makes copies of itself –
self-replication.

The “RNA world” hypothesis
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life may have used RNA as material for both genes and replication
before the emergence of DNA and proteins.

Once an information system can make copies of itself, natural
selection kicks in. Some of the new copies of these molecules (which
some would call “genes”) will have errors, or mutations, and some
of these new mutations will improve the replication ability of the
molecules. Therefore, over time, there will be more copies of these
mutants than other molecules, some of which will accumulate further
new mutations making them even faster and more abundant, and so on.

Eventually, these molecules probably evolved a lipid (fatty) boundary
separating the internal environment of the organism from the exterior,
forming protocells. Protocells could concentrate and organise better
the molecules needed in biochemical reactions, providing a contained
and efficient metabolism.

Life on repeat?

Abiogenesis could have happened more than once. Earth could have
birthed self-replicating molecules several times, and maybe early life
for thousands or millions of years just consisted of a bunch of
different self-replicating RNA molecules, with independent origins,
competing for the same building blocks. Alas, due to the ancient and
microscopic nature of this process, we may never know.

Many lab experiments have successfully reproduced different stages of
abiogenesis
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proving they could happen more than once, but we have no certainty of
these occurring in the past.

A related question could be whether new life is emerging by
abiogenesis as you are reading this. This is very unlikely though.
Early Earth was sterile of life and the physical and chemical
conditions were very different. Nowadays, if somewhere on the planet
there were ideal conditions for new self-replicating molecules to
appear, they would be promptly chomped by existing life.

What we do know is that all extant life beings descend from a single
shared last universal common ancestor of life (also known as LUCA
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If there were other ancestors, they left no descendants behind. Key
pieces of evidence support the existence of LUCA. All life on Earth
uses the same genetic code, namely the correspondence between
nucleotides in DNA known as A, T, C, and G – and the amino acid they
encode in proteins. For example, the sequence of the three nucleotides
ATG always corresponds to the amino acid methionine.

Theoretically, however, there could have been more genetic code
variants between species. But all life on Earth uses the same code
with a few minor changes in some lineages. Biochemical pathways, such
as the ones used to metabolise food, also support the existence of
LUCA; many independent pathways could have evolved in different
ancestors, yet some (such as the ones used to metabolise sugars) are
shared across all living organisms. Similarly, hundreds of identical
genes are present in disparate live beings which can only be explained
by being inherited from LUCA.

[Image of a chimp holding its ears.]

Just like us… Sharon Morris/Shutterstock
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My favourite support for LUCA comes from the Tree of Life. Independent
analyses, some using anatomy, metabolism or genetic sequences, have
revealed a hierarchical pattern of relatedness that can be represented
as a tree. This shows we are more related to chimps than to any other
living organisms on Earth. Chimps and we are more related to gorillas,
and together to orangutans, and so on.

You can pick any random organism, from the lettuce in your salad to
the bacteria in your bioactive yogurt and, if you travel back in time
far enough, you will share an actual common ancestor
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This is not a metaphor, but a scientific fact.

This is one of the most mind boggling concepts in science, Darwin’s
unity of life. If you are reading this text, you are here thanks to an
uninterrupted chain of reproductive events going back billions of
years. As exciting as it is to think about life repeatedly emerging on
our planet, or elsewhere, it is even more exciting to know that we are
related to all the life beings in the planet.[The Conversation]

Jordi Paps [[link removed]],
Senior lecturer, School of Biological Sciences, University of Bristol,
_University of Bristol
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This article is republished from The Conversation
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the original article
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* Science
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* biology
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* Evolution
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* life
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