The theory of evolution by natural selection has largely been accepted as the mechanism by which organisms can change over many generations. Central to Darwin’s thesis was that all life evolved from a single primordial organism, but the characteristics of this common ancestor are unknown. It was certainly microbial, since the earliest parts of the fossil record (from around 3.5 billion years ago) contain only prokaryotic organisms such as bacteria and archaea. How these life forms – which are simple compared to us, but very complex compared to inorganic structures – could have arisen from non-living material is an enduring mystery, although there are many theories.
Darwin himself saw the origin of species and the appearance of life as two separate problems, and imagined ‘in some warm little pond with all sorts of ammonia and phosphoric salts – light, heat, electricity etc., present, that a protein compound was chemically formed ready to undergo still more complex changes.’
Fossils known as stromatolites preserve many-layered colonies of simple microorganisms from up to 3.5 billion years ago.
The most famous theory concerning the origin of life reflects Darwin’s own suggestion that it emerged from some ‘warm pond’ in the distant past. The ‘warm ponds’ in this case were the first permanent oceans that filled Earth’s basins about 3.8 billion years ago. The theory was given its most significant boost by the 1952 Miller–Urey experiment, carried out by chemist Stanley Miller and astrophysicist Harold Urey at the University of Chicago. They assembled an apparatus named the Lollipop for the disc shape of its central reaction chamber. This chamber was seeded with water and chemicals found in volcanic emissions, such as nitrogen, carbon dioxide and sulphides. The mixture was stirred, boiled, condensed and electrified in a constant loop. Within a day it turned pink, and after a week the researchers found it contained many complex chemicals – cyanide, ammonia and even a simple amino acid. If the experiment was run on a larger and longer scale, the researchers reasoned, it would eventually produce all the chemicals of life.
The first step in the appearance of life must have been a non-living chemical process capable of building the biological chemicals used by life. However, the ‘panspermia’ theory neatly sidesteps this issue (at least for our planet) by proposing that Earth was seeded with biochemicals, perhaps even the first living cells, from space. This idea took shape in the 19th century, and has come under closer scrutiny more recently by astrobiologists, who look for life beyond Earth.
There have been several suggestions for the vessels that carried the life-giving materials to Earth. Might encysted bacteria have been able to reach Earth as microscopic dust blasted through the cosmos? Were biochemicals frozen in the ices of comets that vaporized when they impacted Earth? The Philae lander, which analysed comet ice in 2015, would suggest this is unlikely. So perhaps the strongest candidates are meteorites – could biochemicals and even bacteria have been brought to Earth inside the frozen cores of space rocks?
It is possible that the seeds of life on Earth arrived from space aboard meteorites or comets like this one.
In recent years, the idea of life cooked up in a warm ancient sea has been pushed aside by an alternative that proposes it arose in hot seafloor sediments. Although extreme to us, the chemical-rich sediments around seafloor hydrothermal vents would have been one of the most stable environments when Earth was young: the surface regions were subject to intense solar radiation and dramatic climate changes. So many believe that the step from non-life to life was made in the ocean depths.
But what exactly transforms a chemical into a life form? The answer is ‘autocatalysis’, or being a catalyst that makes itself. A catalyst is a substance capable of removing the energy barrier that prevents a chemical reaction occurring – its presence makes the reaction run almost spontaneously. The first life forms were molecules that could catalyse the formation of an exact copy of themselves from a supply of raw materials. RNA is able to autocatalyse in this way, but it is likely that many simpler chemical life forms came before it.
The process of transition from complex molecules to something we would recognize as a life form today is called chemical evolution. We can imagine sediments brimming with chemicals, some of which were able to make copies of themselves through autocatalysis (see here). These molecules were in competition for the same kinds of raw materials. Some would have been better at building accurate copies than others, and so they multiplied and became dominant – the first form of natural selection.
As the replicators became larger and more complex, copying errors or mutations would appear, helping or hindering each molecule in its battle. At some point, a replicator similar to today’s nucleic acids (RNA and DNA) associated with proteins, which helped in its quest to copy itself and protect the delicate molecule. The proteins were coded into the structure of the replicator, creating a life form akin to a virus (see here). The final step saw the whole assemblage shrouded in an oily membrane to protect its supply of materials – the very first cell.
The first cellular life form to evolve out of the chemical stew around 3.5 billion years ago was the ancestor of the prokaryotes, the Bacteria and Archaea domains that still dominate life on Earth. The third domain of life, the Eukaryota, descend from a later single cell that evolved about 1.5 billion years ago. And surprisingly, this cell did not have one ancestor but several. Some of its organelles (see here), such as the endoplasmic reticulum, evolved from folds in the cell membrane, but theory suggests that others – the crucial mitochondria and chloroplasts – came from prokaryotes living in symbiosis inside a larger cell. It is possible that this process of ‘endosymbiosis’ occurred many times, but all of today’s eukaryotes are descended from a single victorious cell. The earliest endosymbiont was probably a sulphur-eating bacteria that evolved into mitochondria (these organelles still carry their own supply of DNA left over from when they lived free). Chloroplasts probably joined later, originating as independent photosynthetic ‘cyanobacteria’.
Short for ‘evolutionary development biology’, evo-devo seeks to bridge the gap between differences in DNA (genotype) and anatomy (phenotype). In doing so, it is a powerful tool for finding the major branches of the tree of life, helping to fill in vast gaps in the fossil record. Instead of simply comparing DNA or anatomy, evo-devo compares the way organisms develop from a zygote, through an embryo into a mature form (see here), and assumes that organisms that develop in the same way early on are more closely related than those that follow a different path.
Evo-devo has been at the forefront of evolutionary biology for the last 30 years, and has shown that body complexity and gross anatomical features are not necessarily good indications of an evolutionary relationship. For example, flatworms and segmented worms are related to molluscs, while crustaceans and insects are more closely related to roundworms (nematodes). Meanwhile, starfish have turned out to belong on the same branch of the tree of life as vertebrates.
In the early stages at least, all vertebrate embryos develop in the same way.
A prime example of the power of genetics, Hox genes are the ‘source code’ that controls embryonic development in most animals Often characterized as a ‘developmental-genetic toolkit’, they are used in all Bilateria, the ‘subkingdom’ of animals that grow bodies with bilateral symmetry at some point in their lives. Thus, Hox genes are shared by organisms ranging from flatworms to fin whales.
These genes produce an embryo with a head at one end and an abdomen (generally with a tail-like structure) at the other. There are genes for every body segment in between, tagging different parts of the head-tail axis with specific proteins. These chemical tags result in the right anatomical feature growing at each location – perhaps a limb, other appendages or an eye. Hox genes are so important that natural selection has left them more or less identical across the Bilateria, performing the same role everywhere. Some may be supressed in certain species – such as limb genes in snakes – but the basic body plan is always retained.
