My father-in-law has raised all sorts of orphaned animals on his farm, from wallabies to stray puppies. He has vowed to never again raise an emu. Emma-Steve’s initial cuteness deceived us, and even though we knew what sort of monster the chick would grow to be, we seemed to forget that in our desire to save the young chick’s life. Juveniles and adults of a species can be very different, but despite the challenge that development poses to palaeontologists, their careful study of the fossil record, coupled with the insights that geneticists studying DNA have produced, means we have a good history of life on Earth.
LUCA was a simple bacterium that used the same genetic code that we rely on but was otherwise very different from us. Attempts to work out what its genome looked like suggest that it probably ran its metabolism on hydrogen sulfide produced by volcanoes rather than by eating plants, fungi and animals. Oxygen would have killed LUCA, while we die without it. Its genome would have been tiny compared to ours, and sex was alien to it, while it is necessary for us to reproduce. There were very many mutations, some of which led to novel phenotypic traits, which helped our ancestors survive and reproduce, that were required for you and me to evolve from LUCA, and these played out over nearly 4 billion years. The construction of the tree of life through the study of DNA and fossils, coupled with key events in the history of our planet such as meteor impacts and periods of extreme volcanic activity, has allowed biologists to work out what had to happen for LUCA to evolve into humans. There is still much to learn, but we know a lot. The process of mutation impacting development with natural and sexual selection operating on the resultant phenotypic traits led to us. The molecular mechanisms involved are often very complicated and are beyond the scope of this book, but I will briefly consider a few of the most important evolutionary innovations on the journey from LUCA to us.
As life secured its footing on our planet it became adept at using a wider range of energy sources than the hydrogen sulfide that LUCA likely relied upon. By at least 2.7 billion years ago, and perhaps as long as 3.5 billion years past, life had found a way to run its metabolism using light. The first bacteria to do this were cyanobacteria, the type of bacteria that produce the stromatolites found at Shark Bay in Western Australia and in the Bahamas. They use photosynthesis, something that all plants, algae and many bacteria rely upon today.
Photosynthesis uses the energy from photons to break apart molecules of carbon dioxide and water before using the freed carbon and hydrogen atoms to build the amino acids needed to make proteins. In doing this, oxygen is produced as a waste product. Although each individual cyanobacterium produced very little oxygen, with billions of them releasing it into the environment over millions of years, all the elements in the Earth’s crust and atmosphere that could react with oxygen eventually became oxidized, at which point oxygen concentrations began to increase in the atmosphere. Unreacted oxygen first appeared in the Earth’s atmosphere 2.33 billion years ago, steadily increasing in abundance over the next few million years until it constituted 3–4 per cent of the atmosphere. This is only a fraction of 21 per cent of the atmosphere that oxygen constitutes today.
The cyanobacteria were the first global polluters, and although life went on to use the oxygen they produced to great effect it was poisonous to most forms of early life. Billions of bacteria were killed by cyanobacteria’s oxygen. But one species’ poison is another’s meat, and life found a way of using oxygen. The endless puffs of oxygen from countless cyanobacteria billions of years ago paved the way for you and me to exist. The great oxygenation event in which oxygen increased to 3–4 per cent of the atmosphere was a necessity for our evolution, but it did not foretell it.
An increase in oxygen in the atmosphere allowed a new type of metabolism to evolve, with organisms adapting to use the reactive nature of oxygen to gain energy from glucose, a type of sugar that is used by a very wide variety of life forms. As evolution has shown time and again, when a new resource becomes sufficiently abundant, life finds a way to use it. Once life had mastered how to use oxygen to break down glucose it changed the world beyond recognition, allowing new ways of life and organisms built from more than one cell to evolve.
All animals alive today run their metabolisms off glucose sourced by breaking down large molecules called carbohydrates. Life uses carbohydrates to store energy that can be used at a later date, and for building other large molecules. Every cell contains carbohydrates, so the ability to run metabolism and DNA replication using glucose as a fuel opened up the opportunity for life to thrive by feeding off the living or recently dead. These lifestyles began with single-celled organisms feeding on one another, eventually via endocytosis, but led to predation, herbivory, parasitism and scavenging. Oxygen was consequently a double-edged sword. Not only was it a poison for many bacteria, it also heralded the arrival of a more dangerous world, as the world now contained a new way to die: death by other organisms.
As oxygen levels increased, cells evolved large sizes as more oxygen could diffuse deeper into them. These larger cells required more organizational structure than bacteria and archaea to run efficiently, and internal membranes evolved to help direct and shuttle chemicals around within the cell. These early eukaryote cells could be thousands of times larger than bacteria. Research into the evolution of eukaryotes is a highly active field, with many hypotheses proposed as to how the structures in eukaryote cells evolved. Currently we are not entirely sure how archaea evolved into eukaryotes, but there is one key event worth mentioning. At some point shortly after eukaryotes emerged approximately one and a half billion years ago, a failed predation event occurred when an early eukaryote cell attempted to consume a bacterium. The bacteria survived and it ended up setting up home inside the cell, where it divided to produce offspring. Over time, the early eukaryote and the bacteria evolved a highly effective symbiosis where the early eukaryote provided safety and glucose to the bacteria in return for it running the cell’s metabolism. The bacteria generated the adenine triphosphate, or ATP, that is the end-product of metabolism and is the fuel used to run the chemical machinery in all cells.
Descendants of these bacteria are found in all eukaryote cells, and they are called mitochondria: organelles with their own membranes and DNA. Mitochondria can no longer survive outside eukaryote cells, and eukaryote cells die without them. When our mitochondria cease to function, we die. Something else normally kills us first, but a key reason we age is because our mitochondria wear out. Sometimes the DNA in mitochondria experiences a mutation, meaning they do not work appropriately. When that happens it usually results in an early death. All eukaryotes, including plants and fungi, have mitochondria, with plant cells hosting the descendant of another symbiotic prokaryotic in their cells. These organelles are called chloroplasts, and this is where photosynthesis happens, with light being used to produce the glucose used to run plant metabolism. All complex life on Earth today owes its existence to mutualisms between bacteria and early eukaryote cells that were closely related to strain MK-D1.
Some eukaryote species, including humans, have sex to reproduce. Sex evolved quite early in eukaryote evolution in single-celled organisms similar to yeast. However, not all eukaryotes reproduce by sexual reproduction. In some species, females produce genetic clones of themselves, making males redundant, and this poses a paradox. Every time a clonal female divides, she produces an exact genetic copy of herself. Furthermore, every daughter can produce genetic clones herself. In contrast, every time a sexually reproducing female breeds, on average, only half of her offspring will be female and able to give birth to new descendants; the other half are male and they cannot give birth to new descendants. Each offspring born to a sexually reproducing female also has only 50 per cent of her genome, represented by one chromosome in each chromosome pair. All else being equal, genes in the asexually reproducing clonal strategy would spread through a population at twice the speed of the sexually reproducing strategy. Sexual reproduction would be outcompeted by asexual reproduction, and the sexually reproducing strategy would be driven extinct. Sex is consequently evolutionarily costly, so for it to evolve, there must be some advantages.