The Story of the Universe and Me: Where Did I Come From?
Part 4: Life
How, exactly, the complex biochemistry that animates our cells came to be remains mysterious. The best guess we have is that RNA, ribonucleic acids, were the first part to materialise. These advanced molecules are self-replicating and can assemble proteins, which are the basis for all biological processes. Like its bigger brother, DNA, RNA can carry information in the order of its bases (the rungs of the ladder in those diagrams you’ve seen). A different order of bases means the RNA is able to put together a different matching protein; and all of these proteins have a different behaviour, a different purpose. I use that word, purpose, because it was at this time that ‘purpose’ was first coming into existence.
Around 4 billion years ago, RNA was dominant. It would have both carried around the genomes of the earliest organisms and built the proteins that put them together. RNA strands that could assemble proteins which built protective barriers or abetting helpful reactions were more likely to survive and continue to replicate. Those with less helpful structures died out. Thus, very early life emerged: a collection of chemical processes guided by RNA, able to replicate itself. However, this type of life was short-lived. The RNA molecules were unstable, and broke apart and recombined at random. Although this allowed for a great deal of variety, it prohibited any serious complexity. RNA is very good at giving instructions, but not very good at storing them, like a charismatic dictator with a poor memory.
So DNA, which RNA is able to read and convey information from, began to appear. This substance, although very similar to RNA, is far more stable. It allowed for more complicated, and just plain more, instructions to be stored. So now RNA simply transcribed the information held by DNA instead of storing it itself, and used it to create proteins, which then assembled the much-needed machinery of those first cells. This is a most successful system, and goes on within all one hundred trillion of your cells to this very day.
DNA, too, can replicate itself. By virtue of the machinery built by its servant RNAs and proteins, the best DNA could now do so within the safety of a cell membrane, which could also split and take the replicated strands with it.
Now there was an explosion of diversity. From the first DNA-driven cell (which, by the way, is your earliest ancestor, and mine too, and that of the spider on your wall and the bacteria under your nails), random changes in the DNA sequence of its progeny allowed a whole host of traits and improvements to come about. Any of those traits that increased a cell’s chance of survival, and hence reproduction, remained, while cells that gained unhelpful traits would die and fail to reproduce. In other words, natural selection was taking place, as it already had been in the early RNA world. Some evolved the ability to consume and digest other cells for energy and materials. Others evolved flagella for outrunning those cells. Different varieties adapted to different chemical environments and different roles, and primitive ecosystems began to form. Such was life.
Roughly 3.4 billion years ago, some organism gained the ability to derive energy from sunlight, whereas previously all energy had come from chemical reactions. Photosynthesis was more efficient and made use of a much more abundant energy source. The means by which light was converted into cellular energy also had an incredibly useful by-product: molecular oxygen. Photosynthetic organisms, at that time just cyanobacteria, spread across the planet and filled the air with oxygen.
Up until that point, life had not needed oxygen to survive, instead respiring by sulphur or other molecules. But oxygen is a much more efficient benefactor, and some organisms adapted to make use of it. These organisms became wildly successful. A great many organisms went extinct because they were unable to cope with the newly oxygenated environment.
Around two billion years ago, some cells developed internal membranes, separating their DNA and other components from each other. These cells were not adapted to respire oxygen. Yet.
One member of this group was at some point invaded by a parasitic microorganism which did use oxygen. Somehow, the invaded cell neutralised the attack and survived. The invader, likewise, was able to resist digestion and continued to exist inside its new host. The two cells, quite by accident, now found themselves in a mutually beneficial arrangement. The invader continued to respire using oxygen, producing ATP (adenosine triphosphate, a very efficient energy-carrying molecule), which the larger cell was able to use as fuel. The invader also continued to reproduce, so that the host was soon home to a number of the creatures. They lived in it as it, too, reproduced. Natural selection took full advantage of the curious circumstance, and both the hosts and the things living inside them (‘mitochondria’) were able to adapt to the new living situation. The hosts benefitted by receiving useful energy, converted by the mitochondria from other forms. The mitochondria were well-protected and provided with a nutrient-rich environment to live in. The two became increasingly reliant on each other until they may as well have been a single organism. The descendents of this chimera are widespread today, and include, once again, us. There are about one hundred mitochondria in every one of your cells, powering everything you do!
(A similar story occurred later on, when one of those cells descended from the chimera also swallowed a photosynthetic cyanobacterium. The cyanobacteria became chloroplasts, acting as the cell’s source of energy, which the mitochondria also living in the cell converted the energy to ATP. Descendents of this union include all plants.)
Although it is still unclear when exactly this took place, another trait had already emerged: sexual reproduction. This would soon become the basis of all complex life on the face of the planet.
To be continued!