The Origin of Life on Earth
More profound than the question “Are we alone?”
may be the question “How did we get here?”. Our natural curiosity as humans leads us to
deeply philosophical questions, and science is a tool we use to reveal the
answers. However, when it comes to
piecing together the mystery of how life arose on Earth, we are still far from
a complete answer. Investigating the
origin of life on Earth is not a trivial endeavor. The biological and geological histories of
Earth are intimately entwined, not separate tapestries waiting to be
unraveled. Uncovering the story of the
origin of life, therefore, requires the collaboration of scientists from many
different disciplines.
The origin of life on Earth represents the very
beginning of biology on our planet. To
better understand the events leading to the origin of life, we have to examine
more than just the biology. We must
carefully consider the environmental conditions of early Earth, the source of
“life essentials” such as organic molecules and water, and the mechanisms that
would have resulted in early cellular life.
Our Earth formed at the same time as all the
other planets in the solar system, about 4.5 billion years ago. The first undisputed evidence for life on Earth
appears in the geologic record approximately 3.8 billion years ago. What was happening during those first few
hundred million years? At the time of
Earth’s formation, the solar system was not a very nice place. Earth was constantly bombarded by planetisimals.
Oceans had not yet formed on the planet and the crust was still
molten. Until bombardment by meteors,
comets, and other space debris slowed, Earth was not habitable. During these first few hundred million years,
Earth was changing significantly; the crust cooled and solidified, the oceans
formed, and the conditions on Earth began to stabilize. Once these things happened, it is thought
that life almost immediately began.
Scientists are unsure exactly what Earth was
like during those first few hundred million years. There are no rocks from this time period
because of plate tectonics and the dynamic nature of Earth’s surface. The oldest rocks identified date to 3.9 billion
years ago. However, there are crystals
of zirconium silicate – zircons – that are 4.4 billion years old. In addition to the lack of information in the
geologic record, there is also no way to know what the atmosphere was like at
that time, although it is certain that Earth was not oxygen rich until a few
billion years ago. Currently, most
scientists agree that our early atmosphere was “mildly reducing” which means it
was dominated by carbon dioxide (CO2) and nitrogen (N2) with small
amounts of carbon monoxide (CO), hydrogen (H2) and reduced sulfur gases. Whatever the actual composition turns out to
be, it is important to note that oxygen was not a key player. Due to the lack of oxygen, some of the first
forms of life on Earth were most likely anaerobic – organisms that do not
require oxygen to live. It has also been
proposed that these first life forms were probably photosynthetic, therefore
releasing oxygen into the atmosphere as a byproduct. The atmosphere would have remained reducing
until enough oxygen was released by living organisms to raise the amount of
oxygen, and eventually ozone, to appreciable levels.
Before life could begin on Earth, the components
for life had to be present. At the very
minimum, the first form of life needed water and organic molecules to create
the cells that we readily associate with living organisms. Again, scientists are unsure how Earth got
its supply of water and organic material.
We have already casually mentioned that Earth’s oceans formed relatively
early on. The isotopic composition of
zircons suggests that liquid water was present on Earth as early as 4.4 billion
years ago. But where did all that water
come from? Earth is one of four
terrestrial planets in our solar system.
Terrestrial planets are characterized by their rocky compositions and
lack of volatiles – including water.
Water is relatively abundant in the solar system, but it is concentrated
in the outer planetary bodies. As a
volcanically active planet, it is likely that water incorporated into Earth
during accretion could have been released into the atmosphere through volcanic outgassing. However,
it is doubtful that sufficient water could have resulted from accretion and outgassing alone.
Some scientists propose that collisions of comets with Earth during the
period of heavy bombardment may have delivered water. Based on what we currently know about comet
composition, this mechanism of delivering water may have only contributed 10%
of Earth’s water. More recent studies of
carbonaceous and ordinary chondrites – some of the
oldest material in our solar system – have caused other scientists to suggest
that meteorites brought water to Earth.
Regardless of exactly how, it is probable that all three of these
methods contributed in some way to Earth’s water reservoir.
What about complex organic molecules, like amino
acids? Stated simply, organic molecules
necessary for life as we know it either originated on Earth, outside of Earth,
or a combination of those two scenarios.
Again, it is difficult to provide a concrete response since we have
limited knowledge of the early Earth.
However, scientists have worked to simulate early Earth conditions in
the laboratory setting. The first of
these scientists were Harold Urey and Stanley Miller in the early 1950s. Miller was the bright young graduate student
of Urey. Together they created an analog
of Earth’s early atmosphere. To this
atmosphere they added energy in the form of a spark, meant to represent
lightning on early Earth. What they
discovered from this simple experiment was that a variety of complex organic
molecules formed readily in these conditions.
Although it did not create all of the 20 amino acids used by life forms
today, it was the first evidence that organic molecules could have been a
natural chemical result on early Earth.
More recently, scientists have been exploring the nature of matter
beyond Earth. It turns out that organic
molecules are not as rare as was once thought.
Indeed, some scientists are suggesting that Earth received its supply of
complex molecules from comet and meteorite collisions with Earth.
Once the conditions on Earth were conducive, it
is thought that life developed immediately.
It is important to understand that “immediately” in this case should be
thought of within the context of geologic time scales and can represent several
thousand to several million years. There
is no doubt that the conditions and materials necessary for the origin of life
were all present. However, there is
great debate about how and where life first formed on Earth. Did the first form of life have DNA, or was
some other molecule used to store information?
How were preliminary cells structured?
How were the first proteins synthesized?
Many scientists agree that DNA might have been too complicated to be the
first information storage molecule.
Instead, RNA or some other molecule may have filled the role until cells
evolved to utilize DNA. No one knows,
but there has been a lot of exciting work examining these first molecules. After examining evolutionary relationships
between organisms, there have also been proposals about where life formed. Some scientists feel that the first life
forms on Earth originated in hot environments, like around hydrothermal vents
or in
At this point there are still many missing
pieces to the puzzle of “how we got here”.
One thing is for certain, the more we understand about the origins of
life on our own planet, the better prepared we will be at predicting where to
look for life elsewhere.