While Darwin and Wallace’s theory of evolution explains the vast diversity of species we see today, their theory doesn’t offer an explanation as to HOW life came to be on our planet 3.2 billion years ago.
We know from looking at extant species today that certain properties of life are maintained and thus have been results of the process of natural selection, i.e. adjusting and balancing the internal environment known as homeostasis, the ability to maintain discrete parts known as structural organization, the ability to control chemical reactions within the internal environment known as metabolism, growth and reproduction, and the ability to actively respond to environmental cues.
Our most basal ancestor – that which underlies the development of all living organisms present today – is known as our Last Universal Common Ancestor (LUCA). LUCA is not a single organism but rather constitutes an entire population of organisms beyond which we cannot see based on our current methods of tracing evolutionary relatedness. To see beyond LUCA, biologists work with chemists, geologists, and atmospheric scientists in order to understand how life emerged on our early planet.
The atmosphere of early earth lacked oxygen, and so ultraviolet light, lightning, cosmic rays, volcanic eruptions, and the heat from our planet’s core may have served as sources of energy which converted gases in the atmosphere and extraterrestrial particles arriving on comets into a whole range of molecules that became the basis for early life. This early mixture of elements and energy lends credence to the idea of a prebiotic soup, or a pool of molecules that existed in a liquid form before life arose that, over time, would have grown richer and more diverse in both living and nonliving matter.
In the 1950s, Stanley Miller and Harold Urey tested this hypothesis by simulating these conditions within their lab. Through their experiments, the scientists were able to produce a number of basic elements needed for life but were unable to describe the mechanism behind the assembly of these components into higher ordered structures. This remained the case until Sidney Fox, in 1977, found that mixing these components (known as amino acids) before added them to boiling water, created bonds similar to those found in modern-day proteins.
The issues of heritability and replication of these molecules was addressed by Sol Spiegelman and Manfred Sumper in the 70’s, and later by Thomas Check and Sydney Altman in the 80’s, showing that single-stranded higher ordered structures derived from the molecules that Miller and Urey and found known as RNA could replicate on their own by using itself as a template, and that natural selection occurred on these copies. In the presence of several different RNA templates and chemical substrates, a large variation of early RNA life was possible, highlighting a period in earth’s history known as the RNA World.
The next jump occurred in the shift from an RNA-driven world to a world dominated by DNA. Scientists hypothesize that evolution selected for double-stranded DNA over RNA over time as DNA is chemically more stable than RNA, and has proofreading and repair mechanisms which lowered mutation rates and allowed for longer genes with more genetic information to be stored. Furthermore, DNA allowed for specialization within cells, where element-rich DNA could be used for genetic data storage while RNA could take on roles of housekeeper within structures we now call the cell.
The formation of membrane-bound cells is intriguing as it highlights the role of early cooperation amongst early life-forms. The hypercycle model proposed that as early elements began to rely on eachother to gather more substrates, their rate of replication became increasingly dependent on one another. Furthermore, this cooperation allowed the elements to persist longer than those that were on their own creating a differential success that natural selection acted on in favouring the replication of all required partners. These early elements also produced fatty-acid which became the cell membrane of the protocell. While the cell membrane posed additional challenges, such as how nutrients would be brought into the cell, it also allowed for a number of benefits: control of the internal microenvironment of the cell, creation of chemical gradients across the membrane allowing some chemicals in while keeping other out, defense against predatory replicators, and the partitioning of various functions in order to operate efficiently. As these cells became increasingly productive, they began to grow, leading to an even split into two parts, or daughter cells.
During early cell evolution, horizontal gene transfer (as opposed to the vertical gene transfer seen between parent and offspring) led to more complex cellular organisms forming, owing largely to the fact that many cellular functions were modular in nature. As cells became increasingly complex in structure and function, that modularity would have decreased as specialization increased, leading to the less important role of horizontal gene transfer as we see today. This is important as we can then conclude that identifying a specific ancestral species becomes impossible as our definition of a species begins to break down and we rely on modern-day genomic analysis to identify the minimum gene set required for the functions of all life on earth to continue.