How did life on Earth begin? It is one of the most basic questions asked by science. And it still remains a mystery.
A fragment of the Murtchison Meteorite, on display at the Smithsonian Museum of Natural History
There are some things we are pretty certain of. The Earth itself formed at the same time as the Sun and the rest of the solar system, about 4.54 billion years ago. At that time, the entire solar system consisted of a large cloud of gas and dust floating in space. Something perturbed that cloud (perhaps the shock wave from a nearby supernova) and caused part of it to increase its density, and gravity then slowly tugged on the rest, pulling it in. This caused the cloud to spin, and as its density and speed increased, the cloud began to flatten out into a disk, with most of its mass in the center. The particles in the disk began to coalesce, first from electrostatic attraction, and later through gravity as the larger masses began to gather up all the smaller ones. The Sun, at the center, became large enough for nuclear fusion to begin in its core, and the solar system, consisting of thousands of small planetoids circling the Sun, was born. Over millions of years, these small planetoids grouped together to form the eight planets (and the asteroid belt) we know today, and the leftovers at the edge of the solar system became Pluto, Eris, and other objects in the Kuiper belt and, beyond that, the comets found in the Oort Cloud.
After the planets formed, they were subjected to the impact of all the small pieces of debris left over from the early solar system, producing a period of intense meteors called the Late Heavy Bombardment. This lasted from about 4.1 billion to 3.8 billion years ago. Most of the craters on the Moon were formed during this time, and Earth would have been subject to the same rate of impacts. During this time, life would have been impossible.
But evidence indicates that life formed quickly after the Bombardment ended. The earliest fossil evidence of life on Earth are stromatolites, layered formations made by bacteria, dating back to 3.45 billion years ago. Fossilized remains of individual bacteria have been found in chert rocks from Australia, dating to 3.47 billion years ago. But the earliest signs of life date even earlier, and consist of chemical signatures. Life uses a great amount of carbon from the atmosphere. But atmospheric carbon comes in two different forms, known as “isotopes”: carbon-13 and carbon-12. These are identical chemically, but have different weights, and when they are used by living cells in biological processes, the lighter C-12 version tends to be used more easily. Life, therefore, has a slightly higher ratio of C-12 to C-13 than atmospheric air does, and this difference can be chemically detected in ancient soil deposits. And this chemical signature has been found in deposits 3.85 billion years old, indicating that life on Earth formed almost immediately after the end of the Late Heavy Bombardment.
At its biochemical core, all life on Earth is the same, indicating that all terrestrial life evolved from a single common ancestor. Life is an interaction of proteins, complex chemicals which carry out all the processes of life. Proteins are made by stringing together shorter chemicals called amino acids. The process of protein production is carried out by DNA; particular sections of DNA called “genes” carry out the production of each particular protein. DNA is itself made from four different chemicals called “nucleotides”. In most Earth life, the process of translating DNA to proteins is mediated by a simpler version of DNA known as RNA; some viruses do not have any DNA and use RNA alone for that job. (Because DNA is dependent upon protein enzymes for its action but RNA is able to catalyze itself without enzymes, it has been proposed that our current “DNA World” was itself preceded by a simpler “RNA World”). All of this biochemical machinery is encased and protected in a “cell”, which consists of several semi-permeable membranes which control the entry and exit of various chemicals from the environment.
This whole process is very complex, and biologists assume that it is itself the end product of a long process of evolution and developed slowly from simpler pieces, gong back to the simplest “life” of all–a naked self-replicating molecule. Explaining how the original self-replicating molecule appeared is the Holy Grail of biochemistry. There are several current hypotheses:
Lightning in the atmosphere. This is the earliest hypothesis about the origin of life, first proposed back in the 1920’s. It got a big boost in 1953, when two researchers named Urey and Miller carried out an experiment in which they passed an electric spark through a flask filled with gases believed to have been present in the early atmosphere. The energy from the spark was enough to produce some forms of amino acid and sugars, the basic building blocks of life, which then built up in the primitive oceans and interacted to form more complex molecules.
Clay particles. Clay is an abundant material that is readily formed by erosion of rocks. Clay also has the property of being made up of many small particles with a huge surface area, which easily attracts and holds other molecules through electrostatic and chemical attraction. On the early Earth, one hypothesis goes, water-soaked clay particles may have attracted complex carbon molecules to their surface and held them there, allowing them to become concentrated, interact with each other, and form ever more-complex forms, eventually leading to a self-replicating molecule.
Sea floor hydrothermal vents. On the ocean floor, places where two tectonic plates are spreading apart are studded with volcanic vents where mineral-rich super-heated water boils up into the sea. These hydrothermal vents contain a soup of different chemicals, and also have the advantage of being protected from the intense UV radiation that must have bathed the early Earth’s surface before the formation of a thick oxygen atmosphere. It has been hypothesized that the thermal energy may have allowed early seafloor vents to produce complex life molecules.
Life from space. One of the difficulties with origin of life hypotheses is that Earth life seems to have appeared so quickly after the Late Heavy Bombardment, leaving little time for such a complex process to have happened. The “space” hypothesis avoids that difficulty by suggesting that the bulk of the evolution of life’s building blocks happened inside dust clouds in outer space, before the Earth even appeared. From spectral analysis, we know that interstellar clouds contain large amounts of carbon, and that these atoms can react with each other using the energy from ultraviolet light. It has therefore been hypothesized that many of the building blocks of life, including amino acids, were actually formed in interstellar space, and subsequently carried to other planets inside comets and asteroids.
The “space” hypothesis got a big boost in 1969, when a meteorite fell to earth near the town of Murtchison, Australia. Because it fell during the day and was seen by thousands of people, the meteorite fragments, about 200 pounds in all, were collected within just days of impact, avoiding the contamination from bacteria that can happen when meteorites sit on the Earth’s surface for a long time.
When the Murtchison Meteorite was examined, it was found to be a type of meteorite called a “carbonaceous chondrite”. It was about 4.6 billion years old, from the time the solar system first began to condense–about a billion years older than the first signs of life on Earth. Unlike most meteorites, which are almost pure nickel-iron, chondrites contain a relatively large amount of carbon compounds. The Murtchison Meteorite was found to contain about 2% carbon material and about 10% water molecules. It also contained something surprising–amino acids, over a dozen forms of them, including glycine, alanine and glutamic acid, all used by life chemistry on Earth. A few unusual varieties like isovaline and pseudoleucine were found that are not used by Earth life (and some amino acids commonly used by terrestrial life, including threonine and serine, were missing in the meteorite).
Researchers in 1997 looked at a property of these molecules called “chirality”, and found something interesting. Many life molecules come in two different versions, known as “left-handed” and “right-handed”. Life preferentially uses the left-handed molecules. It was long assumed that the chirality of any molecules chemically formed in the absence of life would be a random mix of right and left hand, but study of the Murtchison Meteorite showed that there was an abundance of left-hand over right, and therefore some chemical process was operating which preferentially produced the versions that were later used by life. Indeed, the meteorite was a veritable chemical factory–over 10,000 different compounds have been identified inside it. The conditions in outer space seem to be capable of supporting a rich chemical environment and producing complex molecules.
Since then, amino acids have been found inside other carbonaceous chondrites.
So far, no indications have been found that any self-replicating molecules have been formed in the conditions of outer space. But the Murtchison Meteorite does demonstrate that the simplest building blocks of life can form in space and be carried to the surface of planets, where they can undergo modification in places like seafloor vents or clay deposits, perhaps helping to pave the roadway to life.