THE FUNDAMENTALS OF LIFE
SCIENCEPHOTO^ LIBRARY^ X2 ,^GETTY,^ MATTHEWJ^ PARKER/WIKIPEDIA,^ UNIVERSTY^ OF^ WISCONSIN- M
ADISON5 Most scientists say that these
chemicals, at best, simply ‘stocked the
soup’. There is no evidence t hat cells
or more complex biological molecules,
such as protein or DNA, have t ravelled
to Earth from space.
Q
So what was the first biological
molecule?A
The holy grail of origin-of-life
research is understanding
which chemicals developed life-like
properties first and how they began
to work together.
The fact that DNA carries the
instructions for life suggests it was
central to early life. But researchers
are increasingly focused on another
molecule, RNA, as potentially the first
chemical to come to life. RNA is
similar in structure to DNA and
performs lots of key functions in cells,
f rom ma k ing proteins to t ra nslating
and communicating the genetic code.
‘R NA world’ is t he na me given to t he
theory that before DNA, self-
replicating RNA units began to
proliferate and evolved complexity.
Researchers making random
sequences of RNA have found that
some can form complex shapes, which
help them perform various functions,
like being a catalyst for the production
of other molecules. And scientists
have created an RNA molecule (R3C)
t hat helps to create more of itself. This
‘protogene’ lends support to the idea
that chemicals can develop life-like
properties such as self-replication.
Other theories suggest that life
began with a much simpler version
of DNA and RNA – one that was easier
to form from the chemicals of early
Ea r t h. This t hen evolved into t he
amazingly robust and efficient
information-carrying molecules that
we see today.
Prof Nicholas Hud, from the NASA-
funded Centre for Chemical Evolution,
believes there may have been several
biological molecules coexisting at one
point a nd ‘life’ as we k now it sta r ted
when t hey bega n to cooperate. “I don’t
believe that there was one first self-
replicating molecule. I think we are
descendants of the polymers that
started to work together. Four types of
polymer essentially form most of the
metabolism of life: lipid membranes,
polysaccharides [sugars], proteins and
nucleic acids. These are the survivors
of perhaps many different polymers.”Are there any other theories
that are considered possible?A
There are dozens more theories.
Many are based on conditions
that might have helped concentrate
important biochemicals and protect
t hem f rom degradation, such as t he
‘clay theory’, which suggests crystals
in clay helped arrange organic
molecules into organised patterns.
Others attempt to deduce the order
in which the molecules of life formed
and began to cooperate. One example
is the ‘lipid world’ theory, which
suggests that membrane-like bubbles
of fatty molecules were the first step
towards cellular life. Although these
wouldn’t be information carrying
units, like R NA or DNA, t hey may
have been able to produce more of
themselves and RNA might then have
for med more easily wit hin t hem.Q
Will we ever find a satisfactor y
answer?A
Scientists working on this
problem still disagree on the
fundamentals. Speaking to origin-of-
life researchers at times sounds like
they are moving further away from a
consensus, rather than closer.TIMELINE
3.8-4.5 billion
YEARS AGO
First living organisms
appear. Specks of graphite,
thought to be produced by
early life, have been dated
to 4.1 billion years ago – as
old as the oldest rocks.3-3.5 billion
YEARS AGO
The oldest evidence of
actual microbial cells dates
back to around three billion
years ago. These were
similar to cyanobacteria
(below). The last known
common ancestor of all life
on Earth lived in this period,
just before cells split into
two main types:
bacteria and
archaea.2-2.5 billion
YEARS AGO
Evidence of oxygen in the
atmosphere suggests an
abundance of oxygen-
producing, photosynthetic
organisms. The first
eukaryotic cells
appear, thought to
be the result of
one cell enslaving
another as an
energy source.2-3 million
YEARS AGO
The first humans or human-
like beings appear on Earth.580 million
YEARS AGO
Multicellular life on Earth
starts flourishing. The
relatively short period
during which many animals
first appeared is known as
the ‘Cambrian explosion’.
Pictured are some creatures
that lived in this period.Q
In one tweet...
Life may have started 4.1bn years ago, not long
after the Earth formed. But how did it begin?
It’s one of science’s greatest questions.