Abiogenesis is the process by which life arises from inorganic matter. Fossilised primitive bacteria have been found which are 3.5 billion years old, which means life could have existed for much longer. Although, given the age of the Earth at around 4.3 billion years, scientists estimate that life has probably been around for no more than 3.9 billion years on the planet. But what is ‘life’ and how do you distinguish it from ‘inorganic matter’?
Erwin Schrodinger in 1944 defined life as matter which avoids the Second Law of Thermodynamics, which says that entropy (disorder) always increases. Living things resist this entropy by taking in nutrients. Therefore, some scientists have tried to define life in terms of metabolism, the ability to take in energy and use it for activities and then get rid of waste by-products. But this definition faced a problem. In an experiment carried out on Mars in 1976 by the Viking Lander’s Labeled Release Experiment, it was found that inorganic matter also showed signs of having a metabolism. This definition of life was soon rejected.
A more modern, narrow definition of life, which excludes inorganic matter, is the ability to self-replicate. The astrobiologist Benton Clark, from the University of Colorado, says life has three main qualities: being able to reproduce, being able to use energy and having a set of genetic instructions. It seems that the first and third qualities are unique to life. Richard Dawkins has said that the origin of life involved some sort of primitive molecule which could replicate itself, using probably a primitive genetic code. He also says, however, that he, nor anyone, has any idea how life originated. Well, no-one knows for sure, but there are some interesting hypotheses.
The most famous is probably the “primordial soup” theory. Alexander Oparin proposed this theory in his book The Origin of Life (1924) and was later adopted J.B.S Haldane around the same time. Oparin and Haldane argued that in the early Earth, organic molecules could have formed in “primeval soups” using the energy of sunlight. The molecules would grow in increasingly complex ways until they were able to reproduce.
Then in 1952, in the Miller-Urey experiment, a mixture of water, hydrogen, methane and ammonia (Earth’s early chemical make-up) was given electrical sparks (to simulate lightning striking Earth’s early oceans). After only a week, organic molecules were found, including amino acids, which are the building blocks of proteins and essential to life. This famous experiment has since been repeated, yielding even more kinds of amino acids than Milley and Urey had found themselves. It was a great achievement, but amino acids are still a long way off from a self-replicating molecule with a genetic code.
Christian de Duve, the Nobel prize-winning chemist says in his book Vital Dust, that the first forms of life could have been RNA molecules. RNA molecules are essentially a single strand of DNA and are capable of storing information and catalysing (speeding up) chemical reactions. Since short RNA molecules have been easily produced in lab conditions, scientists like Duve have suggested that in the early conditions of the Earth, RNA strands could have joined together, eventually creating a self-replicating molecule. Once this molecule was created, natural selection would take over and prefer the self-replicating RNA over those which cannot self-replicate. So may have been the precursor to DNA as we now know it.
Other scientists have taken a different route and have instead focused on where life could have originated on Earth. In the 1970s Thomas Gold claimed that life might not have begun on the surface of the Earth in some “primordial ocean”, but well below the Earth’s surface. In the 70s it was found that simple organisms (called Archaea) were found living in extreme conditions, such as in hydrothermal vents, which are insanely hot. Therefore, these organisms were called extremophiles, since they seem to thrive under extreme conditions (extreme from our point of view anyway).
It has since been found that these organisms can also thrive in conditions without any oxygen, which is significant because the early Earth’s atmosphere was not made up of any oxygen. Oxygen has only been present in the atmosphere since photosynthetic creatures evolved around 300 million years ago. Some scientists have argued that life could have originated in these hydrothermal vents. Bacteria have also been found a foot below the surface of the Atacama Desert, which is extremely dry and lacking in water. This might mean that bacteria are thriving underneath the surface of Mars! These ideas are exciting, but there is one more hypothesis which is even weirder.
Panspermia theory is the idea that the seeds of life came from outer-space, most likely from another planet. Central to the theory is the idea that life exists throughout the universe and is transported from planet to planet by asteroids and meteorites. Extremophiles play an important role in this theory, since if life is transported across space, then that life must be able to survive the harsh conditions (coldness, lack of oxygen, solar winds, ultraviolet radiation etc.) of space. The idea can be traced back to the 5th century BC. The Greek philosopher Anaxagoras claimed that the universe is made up of an infinite number of “seeds” and that if these seeds reached Earth, they would give rise to fully-fledged life. Anaxagoras coined the term panspermia which literally means “seeds everywhere.”
In the 19th century, Hermann Richter said life has always existed in the universe, which seems to get rid of the need for explaining its origin. Although you could still ask why has life always existed? In any case, panspermia theory gained popularity in the 70s after scientist Fred Hoyle said that some “dust” in the universe was mostly organic, that is, made up of carbon. He also argued that there is evidence of bacteria and virus-like organisms in space, particularly in comets. There are signs from the dust of Halley’s Comet, for example, which indicate the presence of bacteria.
In 1973 Hoyle put forward the even more radical idea of directed panspermia, an idea which Francis Crick (co-discoverer of DNA) has also accepted as an explanation for the origin of life. This is the idea that advanced civilisations had evolved elsewhere (which they probably have) and that the germs of life were transported to Earth on their spaceships. Or it could have been that DNA was a molecule artificially made by an advanced civilisation, given the fact that it is such a complex molecule. Jeremy Narby, for example, in his book The Cosmic Serpent, thinks that because DNA contains such astronomical amounts of information that it can’t have come about through Darwinian evolution. Directed panspermia is a very science-fiction type scenario, but it is still in the realm of possibility.
As evidence for panspermia as a more general theory, Earth was heavily bombarded by meteors 3.8 billion years ago, the same time that life was believed to have originated. Statistics have also shown that 7.5% of rocks from Mars have reached Earth. Astrophysicist Neil de Grasse Tyson has said that billions of years ago when there was water on Mars, life could have been plentiful. So life could have evolved on Mars, then travelled on a piece of its rock to the Earth – it would take no more than 16,000 years for the rock to reach Earth. Neil de Grasse Tyson says we could all essentially be Martians.
Scientists have analysed a meteorite in Antarctica which is 16 million years old. They say it might have been blasted off the surface of Mars and a small group of scientists say that rod-like structures on the rock indicate fossilised bacteria. But others are more sceptical. Water oceans might also exist on moons such as Europa, so if life didn’t travel from Mars, it could have travelled from one of these moons as well. As more evidence for the panspermia theory, researchers in Germany have exposed bacteria to outer space and the bacteria survived the harsh conditions.
Of course, one obvious problem with the panspermia theory is that it still doesn’t tell us how exactly life originated, that is, the process which turned inorganic matter to life. Also, the theory doesn’t tell us exactly where life came from, only that it travels on comets and meteorites, which could travel from anywhere in the solar system or even from anywhere in the galaxy. Others have objected that the conditions in space could never allow bacteria to survive, but the more we learn about extremophiles the less persuasive this criticism becomes.