Life on Earth has been around for a much longer period of time than humans have: so how can we know about what preceded us? A review of the past 4.5 billion years on Earth will be provided in this blog post, before going into the methods of how researchers were able to form such a timeline, only being able to use present-day Earth as their 'research field'.
One of the most contested questions on this theme is how life first arose. There are numerous different mechanisms that have been proposed, however, we are not yet sure (and may never be fully sure) which is correct. One such hypothesis suggests that RNA was the first component that organisms were based on, as although it has the same abilities as DNA in which it is able to carry information, it is also able to fold up into a 3D structure to act as an enzyme. Another suggests that several chemical reactions following one another resulted in funnelling energy from the environment into cellular life. Supporting this, the Miller-Urey experiment successfully showed that a mixture of inorganic gases including methane, ammonium, and hydrogen (which is analogous to the early atmosphere of Earth) could result in the spontaneous synthesis of several carbon compounds essential for life, including several amino acids. Although the mechanisms are still heavily disputed among scientists in the area, there is a definite time window in which this occurred, ranging from 4.5 billion years ago when the Earth was formed through elements colliding and binding together, to 3.4 billion years ago, the earliest confirmed fossils known to scientists. Although there is evidence to suggest that life could have been present up to 4.1 billion years old (through zircons found in Australia), this is still heavily contested around the globe.
Around this time, the Last Universal Common Ancestor (suggested to have a lipid bilayer and used DNA, RNA and proteins) split into two of the three current domains of life, namely Archaea and Bacteria. Throughout this period, mats of archaea and bacteria dominated the oceans, where cyanobacteria (bacteria which were able to harness light energy to produce organic compounds through photosynthesis) began to produce oxygen as a waste product. This led to the Great Oxygenation Event, beginning around 2.4 billion years ago, which poisoned high proportions of anaerobic lifeforms present due to the high concentrations of oxygen present in the atmosphere. At around 1.85 billion years ago, eukaryotes were thought to form from prokaryotes phagocytosing each other: one form of evidence for this theory is that chloroplasts found inside our cells are phylogenetically related to cyanobacteria and mitochondria to Rickettsiales bacteria. In the case of mitochondria, it is thought that an anaerobic lifeform engulfed an aerobic lifeform but did not digest it, which led to a surplus of energy for the anaerobic lifeform, paving the way for more complex life: a similar process occurred to form chloroplasts. This led to multicellular forms of eukaryotes developing around 1.5 billion years ago, where differentiated cells could perform specific functions in the organism as in our bodies today. Over the next billion years, life would diverge and evolve, leading to the formation of plants and animals, although not extremely specialised with organs etc.
542 million years ago, the Cambrian explosion occurred debatably due to an increase in oxygen levels caused by an increase in the population of photosynthetic lifeforms. This then led to increasingly complex lifeforms forming, where predator-prey relationships began to appear. As predators began to become more specialised in finding and assimilating prey, prey adapted to this by finding ways to evade harm, hence why the Cambrian explosion was characterised by animals being able to burrow under microbial mats to evade predators. This diversity boom, as well as other abiotic factors, (including changing mineral concentrations), is the reason why the high majority of life present today evolved from organisms in the Cambrian. The earliest fossil record we have of the colonisation of land dates to 440 million years ago, namely the fungus Tortotubus, although there may have been some simple spore-forming or unicellular forms that spread to land prior to this. Such a form which gained its energy through externally digesting and breaking down other simple organisms helped develop the world's soil, which half a billion years later is one of the most important factors of life. Soon after this, arthropods, earthworms, and invertebrates began colonising the land, leading to the first land ecosystems forming. Soon after, fish began to evolve jaws to break down their prey, as well as diverging to form tetrapods, organisms with four feet. This allowed bigger, often more complex amphibians to begin further colonising the land, so more complex ecosystems formed. Now that organisms had evolved support structures; new respiratory systems; and more controlled methods of reproduction (one of which was the amniotic egg), life had fully established itself on land as well as on water. This period of the late Devonian included the rapid growth and divergence of land plants which generated rapid global cooling due to lowering the CO2 in the atmosphere (because of 'excessive' photosynthesis occurring). It also led to the rapid development of soil on Earth, which increased the weathering of rocks, leading to algal blooms which depleted oceans' oxygen concentrations: this was the Late Devonian mass extinction that occurred 360 million years ago, where 75% of all species died as a result.
Following this, Earth experienced the Carboniferous period, where we saw the evolution of life into amniotes, tetrapod vertebrate animals. Amphibians continued to dominate life around them, reasoning for the name of this period being the 'Age of Amphibians', and in the late Carboniferous, we saw the separation of synapsids and sauropsids, the precursors to mammals and reptiles respectively. Soon after this, we saw perhaps the most devastating loss of life occur during the End-Permian Extinction Event, also known as 'The Great Dying', where a staggering 96% of species were lost. Although the precise reasons for this extinction are not known, scientists have characterised this under intense volcanic activity in Siberia, forming the 'Siberian Traps', which released extremely high levels of carbon dioxide and sulfur dioxide into the atmosphere, leading to oceans being both anoxic and sulfidic, as well as heightened average global temperatures. This event affected marine organisms more than those on land, perhaps due to the euxinic conditions of the ocean, leading to the extinction of genera such as Trilobites, Eurypterids, and Blastoids. Although events such as these, where such a large proportion of species died within such a small period of time, may have led to the eventual diversification of life due to widely different environments and selection pressures appearing, the first tens of millions of years following this involved the rebuilding and recovery of life to its former glory.
Soon after, the first dinosaurs (prosauropods) appeared on the supercontinent Pangea, leading to the rule of the dinosaurs, lasting 174 million years. Contrary to common belief, they were not immediately so diverse and successful, falling short of a different group of archosaurs: the predecessors of crocodiles: it was the End Triassic extinction that once again led to changes in selection pressures which this time favoured the dinosaur. During the Jurrasic and Cretaceous periods on Earth, these early dinosaurs would evolve into three main groups, namely, Sauropodomorpha (long-necked dinosaurs with small heads and thick yet long limbs), Ornithischia (mainly herbivorous dinosaurs that had a pelvic structure similar to birds), and Theropoda (dinosaurs characterised by hollow bones and three toes on each limb), the latter two thought to be more closely related. These groups of dinosaurs dominated the Earth for over a hundred million years, leaving little chance for other groups of organisms to evolve in such a rapid manner as they did. Incomplete data suggested that there was a possibility that several (in particular herbivorous) dinosaur species were going extinct in the run-up to the asteroid impact, perhaps due to intense volcanic activity and tectonic shifts. However, the ending point for this group of reptiles was the long-term aftermath of the infamous 6-to-9-mile-wide asteroid that hit today's Yucatán, Mexico 66 million years ago. The impact was devastating in itself, causing an explosion relative to 4.2 x 10>23 Joules of energy. Shockwaves, tsunamis, volcanic eruptions, wildfires, acid rain, and earthquakes all immediately followed this explosion: dust that flew into the air raised global temperatures so much that any animals that were too large to find some form of shelter underground or in some form a closing (e.g. cave) were immediately burnt. The dust and particles 'kicked up' by the shockwave stayed in the air for numerous years, blocking the sun, and thus leading to the extinction of 76% of species overall due to this event. The only animals that had some chance of passing this catastrophe were those with low metabolisms that could essentially 'wait for the event out', or small organisms that could feed on the high volumes of dead matter available. All non-avian dinosaurs did not fit either category, leading to their unfortunate extinction in the last mass extinction to date.
This leads us to the Cenozoic era, which ranges from 66 million years ago to the present day. Throughout this period, animals have evolved in a relatively uniform fashion (i.e. no major mass extinctions), including perhaps most notably the evolution of homo habilis (the first human species) 2.31 million years ago, and the emergence of modern humans from Africa 300,000 years ago.
So, now that I have given a very simplified review of the literature present on this topic, it is incumbent that we ask the question of how it is possible that we can be so accurate with our estimates. It is evident that we will use fossils to achieve this: any preserved remains, impressions, or traces of past living organisms (mostly imprints of bones, exoskeletons or shells). Now that we have a precise idea of how old certain rock formations are, (in which the fossils are found), new fossils can be simply dated by relating them to something in which its age is already known - this is often the easiest method of dating used currently, as we already have so much information about the order of rock formations throughout time. But for a fossil that does not have any comparisons, or for finding the date of the first fossil, scientists use absolute dating, a method that involves implementing radiometric dating to find out the precise date of an object.
Perhaps the most famous application of this is carbon dating. This works by measuring the ratio of carbon-12 atoms to carbon-14 atoms in the fossilised organism: throughout the organism's life, this ratio will remain constant. However, at the time of death, the carbon-14 will begin to decay (due to its radioactivity), losing half of its mass every 5370 years (its half-life). Due to the fact that the organism will not intake any more carbon, by measuring the ratio of carbon-12 (which will remain constant) to carbon-14 atoms, the ratio evidently increasing with time, and then comparing this to the original ratio, which is known, we can find the date that the organism died. It is important to factor in discrepancies such as the recent decrease in carbon-12 in the atmosphere due to its conversion to CO2, or the increase in carbon-14 due to nuclear explosions, meaning that the ratio of carbon-12 to carbon-14 in the atmosphere (which we use to estimate the ratio in previous years) will not have remained constant throughout the history Earth. Furthermore, due to carbon-14's low half-life in comparison to other radioactive isotopes, it cannot be used to date an organism that ceased to live longer than 60,000 years ago in an accurate manner, due to the lack of carbon-14 atoms that will remain in a fossil that died in a period older than this. Therefore, methods such as Potassium-Argon dating are used on much older fossils, as they use the decay of potassium-40 to argon-40 to understand the ratio of these two isotopes when found in fossils: potassium-40 has a half-life of 1.25 billion years and is found in a lot of rocks and minerals that may surround such fossils. This makes Potassium-Argon dating an extremely suitable method for dating organisms since the start of life 4.5 billion years ago.
In conclusion, understanding the extent to which life on Earth has evolved over the last 4.5 billion years gives us a perception of how complex and intricate the lifeforms of the current age are. It is morally unjust for homo sapiens, who only evolved around 300,000 years ago, to essentially tamper with the process of evolution in such a drastic way, so much that the current extinction rate is up to 10,000 times higher than the background extinction rate of 10% species becoming extinct every 1,000,000 years. Due to many different factors that will be explored in a later blog post, we are causing huge biodiversity losses, leading to the death of organisms whose traits have been honed by evolution over billions of years.
Thank you for reading this blog, and please suggest a topic that you would want me to investigate in the future in the comment section below. Until next time!
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