The origin of life
Tree of life
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How did life appear on Earth?
Why it was not even a question for
most of the human being’s time.
• Some version of “the gods did it”
• Vitalism
• Spontaneous generation
Tree of life
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Spontaneous generation
•The beginning of life forms from inanimate objects has
been accepted for centuries
•In ancient Egypt it was believed that the heat of the Sun
on the sediments of the Nile produced snakes and
crocodiles
•In Middle Ages people accepted without any questioning
that mud, mist, sunlight and air could become frogs,
insects, rats and every type of inferior plant, fungi or
animal.
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Spontaneous generation
Lamb of Tartary. Most people used to believe that animals could grow from plants
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Spontaneous generation
• Even in 1667 Jan Baptist van Helmont, a Flemish
doctor, wrote a recipe to produce mice: it was required
the sweaty underwear of a woman and a handful of wheat
grains into a barrel. “After 21 days the smell changes and
penetrates the wheat grains, turning the wheat into mice”
• In 1668 the Italian doctor Francesco Redi showed how
the maggots in the rotting meat came from flies’ eggs
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Spontaneous generation
• In fact spontaneous generation was not fully questioned
until the experiments of the French chemist Louis
Pasteur in the second half of the ninetieth century.
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Spontaneous generation does
not occur
• Nowadays it is recognized beyond any doubt that
existing living beings come from pre-existing living beings
and that life forms –no matter how simple– cannot be
originated from decaying or decomposing of matter.
• But then… how did the first living beings come to
existence?
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Something happened in
between
This is the aspect of the Earth
4000 million years ago. The
impacts were so violent and so
continuous that the crust was
totally burning liquid lava.
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This is one of the oldest rocks we
can find on Earth. It was formed
3500 million years ago. The
parallel lines are fossils of
cyanobacteria.
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The first living beings
These wavey patterns could be 3.700 million-year-old fossils.
They were discovered in August 2016.
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Today stromatolites
These round formations are stromatolites. In swallow waters
the cyanobacteria grow in layers and produce a typical pattern
of deposits of CaCO3 .
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The first living beings
Since the 19th Century,
biologists have known that all
living things are made of
"cells": tiny bags of living
matter that come in different
shapes and sizes. Cells were
first discovered in the 17th
Century, when the first modern
microscopes were invented,
but it took well over a century
for anyone to realize that they
were the basis of all life.
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The first living beings
In April 2016, scientists
presented an updated version
of the "tree of life": a kind of
family tree for every living
species. Almost all of the
branches are bacteria. What's
more, the shape of the tree
suggests that a bacterium was
the common ancestor of all life.
In other words, every living
thing – including you – is
ultimately descended from a
bacterium.
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The first living beings
This means we can
define the problem of
the origin of life more
precisely. Using only
the materials and
conditions found on
the Earth over 3.5
billion years ago, we
have to make a cell.
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But hold on for a moment, please
We have just said that
“Using only the
materials and
conditions found on
the Earth over 3.5
billion years ago, we
have to make a cell.”
And what if life did not
begin on the Earth?
What if life came from
outside in the space?
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Panspermia!!
In 1903 the Swedish
chemist Svante
Arrhenius made
popular the idea that life
could have come to the
Earth from somewhere
out in space.
Comets and meteorites
could have done the
job. Did he have a
proof? Not a single one.
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Panspermia!!
It seems difficult to
take seriously such
idea and panspermia
has been long
forgotten until…
Evidences are
suddenly falling from
the skies!!!
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Evidences of Panspermia
•The interstellar dust is
mostly made of large
organic molecules
• Many different
bacteria have been
discovered above 41
km in the atmosphere
• Streptococcus mitis in
Surveyor 3 survived 3
years in the surface of
the moon
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Evidences of Panspermia
•Lichens and even
tardigrades have
resisted the space
conditions out the ISS
• There could be fossils
in ALH84001
• Uracil and xanthin in
the Murchison meteorite
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Types of Panspermia
• Radiopanspermia refers to single spores or
seeds driven by the radiation pressure from stars
• Lithopanspermia refers to spores or cells carried
inside comets or meteorites
• Molecular panspermia or pseudopanspermia
refers to the traveling of organic molecules
instead of cells or spores
• Directed panspermia concerns the deliberate
transport of microorganisms in space, sent to
Earth to start life here, or sent from Earth to seed
new planeary systems with life
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Supporters of Panspermia
• Does it sound like crazy to you?
• Here you have a list of some scientist in favor of
panspermia:
– Fred Hoyle
– Francis Crick
– Stephen Hawkins
– Svante Arrhenius
• It’s not only that all of them won the Nobel prize
in their specific areas, but that they have proved
to be right in the past when they prophesize
events to occur
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The panspermia inconvenient
Even if the supporters
of panspermia are right,
and life began on Earth
as a consequence of
spores, cells or seeds
form outer space this
does not solve the
mystery of the origin of
life because if life came
from outside how did it
started in the first
place?
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An old (and wrong) idea: Vitalism
Before 1800s most people believed in
vitalism. It is the intuitive idea that living
things were endowed with a special,
magical, property that made them different
from inanimate objects.
The Bible says that God used “the breath of
life” to animate the first humans. And the
immortal soul is a form of vitalism.
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A wrong idea: Vitalism
In the XIX century chemists showed how
living things were not made of special
materials or special types of energy.
Despite of that, scientists were still reluctant
to look for a deity-free explanation of life.
Perhaps they were still too emotionally
attached to vitalism.
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Always ahead of times: Darwin
In 1859 Charles Darwin showed how all
the present living beings were all
descendants of a primordial organism that
lived millions of years ago: the last
universal common ancestor.
Darwin and his idea came under a
ferocious attack because it contradicted
the Bible.
Darwin did not dare to write in his book
where this first organism came from.
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Always ahead of times: Darwin
Darwin knew that it was a profound
question, but – perhaps wary of
starting yet another fight with the
Church – he only seems to have
discussed the issue in a letter
written in 1871. His excitable
language reveals that he knew the
deep significance of the question:
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Always ahead of times: Darwin
"But if (& oh what a big if) we
could conceive in some warm little
pond with all sorts of ammonia &
phosphoric salts,—light, heat,
electricity &c present, that a protein
compound was chemically formed,
ready to undergo still more complex
changes..."
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Abiogenesis: the primeval soup
In 1924, Alexander Ivanovich Oparin
published his book The Origin of Life. In
it he set out a vision for the birth of life
that was startlingly similar to Darwin's
warm little pond.
4000 mya the crust was a mess of semimolten rocks with constant volcanic
activity. The gases of the volcanoes
formed the ancient atmosphere and they
reacted due to the energy of storms and
solar radiation. Simple organic molecules
were formed.
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Abiogenesis: the primeval soup
According to Oparin, as the Earth cooled
down intense continuous rain produced the
primitive seas and organic molecules
accumulated on them (he called them
“primordial soup”).
Some chemicals would react with others to
produce more complex compounds. Oparin
thought that molecules central to life would
have appeared in the water.
But some organic chemicals do not
dissolve in water and form spherical
globules in aquatic environments .
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Abiogenesis: the primeval soup
Some of the spherical globules would
have incorporated inside the complex
organic molecules that would have
continue with the chemical reactions.
Oparin called this globules “coacervates”.
Coacervates can take substances from
the surrounding water and also they can
divide.
Oparin proposed that coacervates were
the ancestors of the first real cells that he
called “protobionts”.
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Abiogenesis: the primeval soup
In 1929, the English biologist J. B. S. Haldane
independently proposed some very similar
ideas than those of Oparin.
Just like Oparin, Haldane outlined how organic
chemicals could build up in water, "[until] the
primitive oceans reached the consistency of
hot dilute soup". This set the stage for "the first
living or half-living things" to form, and for each
one to become enclosed in "an oily film".
The idea that life formed in a primordial soup of
organic chemicals became known as the
Oparin-Haldane hypothesis.
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Abiogenesis: Miller’s experiment
In the earliest 1950s Harold Urey was giving a
lecture on the Oparin-Haldane hypothesis. He
stated that, more than probably, there wouldn’t
be oxygen in the primeval atmosphere.
In the audience a young student, Stanley
Miller, asked the professor and Nobel prize if
someone had tried to show the abiogenesis
processes in the laboratory.
Urey, a renowned chemist that had worked in
the Manhattan Project and an activist for the
civilian control of nuclear energy, got
impressed.
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Abiogenesis: Miller’s experiment
So, in1952 Stanley Miller began the most famous experiment
on the origin of life ever attempted.
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Importance of Miller’s experiment
In only on week, Miller obtained several
amino acids and other bio-molecules very
important for living cells. The results were
published in Science in 1953.
It showed that it is possible, and easy, to
obtain the building blocks of life from
simple and abundant substances that
could be present in the primeval
atmosphere in the conditions supposed
for the Earth of 4000 millions years ago.
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Life is more complicated
This very same year, 1953, in Nature, Watson y Crick
published the DNA structure.
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Differences of approach
Suddenly the origin of life attracted a lot of interest
from many different scientists. They began to work on
very different fields depending on their favorite
characteristic of life.
Genetics first
They focus on the
ability of the cells to
dividide passing
information to their
descendants.
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Metabolism first
Cells need to
use a source of
energy to
perform the vital
functions.
Compartmentalisation first
Cells are separate from the
environment by biological
membranes.
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Genetics first
The first living beings had to do copies of themselves
for the ability of replication seems to be central for life.
This first replicator would probably be RNA because it
has been found that some RNA molecules can also
catalyze chemical reactions working as “ribozymes”.
In 1986 Walter Gilbert proposed that life began in the
"RNA World".
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Genetics first
But no self-replicating RNA had been found, and nobody
could figure out how RNA formed in the primordial soup.
This is the reason why other replicators have been
proposed:
• Polyamide Nucleic Acid, or PNA: In 2000 Miller repeated
his famous experiment and obtained the polyamide
backbone of PNA.
• Threose Nucleic Acid, or TNA: 2000.
• Glycol Nucleic Acid, or GNA: 2005.
The alternative nucleic acids might solve the latter problem,
but there was no evidence they ever existed in nature.
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Metabolism first
Before an organism can reproduce, it has
to be self-sustaining. It must keep itself
alive. After all, you cannot have kids if you
die first.
First, you must obtain energy; say, from
energy-rich chemicals like sugars. Then
you must use that energy to build useful
things like cells.
This process of harnessing energy is so
utterly essential, many researchers
believe it must have been the first thing
life ever did.
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Metabolism first
Life would be an organized set of chemical
reactions and minerals would provide the
materials and the catalytic power to catch and
use the energy required for self-organization.
The ideal place for the beginning of life would be
the hydrothermal vents at the bottom of the
oceans. The rocks of alkaline vents were porous:
they were pocked with tiny holes filled with water.
These little pockets would act as "cells". Each
pocket contains essential chemicals, including
minerals like pyrite. Combined with the natural
proton gradient from the vent, they were the ideal
place for metabolism to begin.
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Compartmentalisation first
Reproduction or metabolism cannot
take place in abstract. First, a container
is required to keep all the essentials of
life together.
Any fatty or oily substance will form
blobs or films in water. These
chemicals are collectively known as
lipids, and the idea that they formed the
first life has been called the "Lipid
World".
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The “everything first” hypothesis
In 2001, Szostak and Luisi, writing in Nature, argued that it
should be possible to make simple living cells from scratch, by
hosting replicating RNAs in a simple, fatty blob.
Three years later they found out that using montmorillonite, a
type of clay, the blobs formed hundred of times faster and
they could absorb RNA molecules.
The “everything first” hypothesis
brings a radical new idea about
the origin of life, namely that all
the key components of life could
be formed at once.
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The “everything first” hypothesis
John Sutherland imagines small rivers and streams trickling
down the slopes of an impact crater, leaching cyanide-based
chemicals from the rocks while ultraviolet radiation pours
down from above. Each stream would have a slightly different
mix of chemicals, so different reactions would happen and a
whole host of organic chemicals would be produced.
Finally the streams would flow into a volcanic pond at the
bottom of the crater. It could have been in a pond like this that
all the pieces came together and the first protocells formed.
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The "Hodge-Podge World"
That means we are approaching one of the great divides in
human history: the divide between those who know the story of
life's beginning, and those who never could.
Some of the people alive today will become the first in history
who can honestly say they know where they came from. They will
know what their ultimate ancestor was like and where it lived.
This knowledge will change us. On a purely scientific level, it will
tell us about how likely life is to form in the Universe, and where
to look for it. And it will tell us something about life's essential
nature. But beyond that, we cannot yet know the wisdom the
origin of life will reveal.
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