(Abbreviations: bya - billion years ago; mya - million year ago)
The first trace of life, stromatolites, are assumed to have been formed on Earth around 3.7 bya [1]. They were sticky mats of microbes that were trapped in between sediment layers. The minerals they produced precipitated inside the layers, creating solid structures as the microbes died. They have left their trace on the surface of rocks, a type of carbon molecule that is produced only by living organisms. Stromatolites have been found in a World Heritage Area in Australia, amongst other locations. Researchers from the University of Tasmania, Australia discovered an unusual type of swamp which occurs only in peaty soils, while surveying peat-bound wetlands [2].
Stromatolites. Image source: https://www.livescience.com/57942-what-was-first-life-on-earth.html
Bands present on Stromatolites. Image source: http://www.fossilmuseum.net/Tree_of_Life/Stromatolites.htm
All of these microbes were anaerobes, meaning they could live without oxygen. They gained the energy required by using other electron acceptors like nitrate, fumarate and hydrogen sulphide. However, these molecules produced less energy relative to that produced by oxygen. Eventually, the electron acceptor in the cell became oxygen.
The first appearance of oxygen in the atmosphere was around 2.4 bya, when cyanobacteria (cyano= blue, because of the pigment present in them) evolved to become Earth’s first photosynthesizers. These cyanobacteria are aerobes, meaning they need oxygen to survive. These microbes dramatically increased the amount of oxygen present in the atmosphere, so the anaerobes quickly died out. This event is called the Great Oxidation Event because the atmosphere changed from a reducing atmosphere to an oxidizing one. These were understood from the changes recorded in the seafloor rocks. When oxygen is present, iron gets oxidized and is removed from the environment. Rocks dating before this event have iron bands present, but there are no such bands on those developed after this event [3]. American biogeologist, cosmologist and palaeontologist Preston Cloud, has vastly contributed to the understanding of the Great Oxidation Event. Most of the ongoing research is based on his contributions.
Preston Cloud. Image source: https://www.nap.edu/read/4894/chapter/4#43
Iron bands on sedimentary rocks. Image source: https://www.nbcnews.com/id/wbna45035196
As the amount of oxygen in the atmosphere was increasing, microbes began to live inside the cells of other microbes so that they could be mutually benefitted. The mitochondrion is an organelle present in all animal and plant cells; it is believed to be descendants of these early mutually benefitted microbes. Plant cells also contain chloroplasts. They are believed to be descendants of cyanobacteria that merged with other bacteria. This eventually led to the origin of eukaryotes.
These cells began living together in groups to function more efficiently by diving the work between them. In this arrangement, certain cells did only a specific job. This led to the development of a variety of functionally specialized cells, or tissues, in multicellular organisms.
The first DNA evidence was obtained from sponges dating back to around 800 mya. The oxygen levels were still low but sponges can tolerate low oxygen concentrations. These organisms feed while still being rooted to the ocean beds and consist of just layers of cell and water cavities without much active movement. They also contain hard skeletons, so they became the first reef builders on Earth.
One of the most important periods in the geological time scale was the Cambrian period when hard shells and skeletons were observed for the first time. Shells and skeletons are a great leap in terms of evolution because it provides protection from other organisms, thus establishing the prey and predator relationship.
Around 580 mya, during the Ediacaran period (just before the Cambrian), there was a proliferation in the abundance of sea animals alongside sponges. These looked nothing like modern animals.
Dickinsonia costata fossil. Image source: http://www.ediacaran.org/dickinsonia.html
An artistic impression of Dickinsonia costata in its natural environment.
Image source: https://eartharchives.org/articles/dickinsonia-is-an-animal/index.html
The fossil evidence has been found on sedimentary rocks. The Ediacaran period is named so because more than 1500 well-preserved organisms were found in the Ediacaran Hills in South Australia. These animals are known for their soft bodies and thus, were precursors to the shells and skeletons found in the Cambrian period. Evidence from the Ediacaran period shows that the life forms of that time were highly diversified was earlier than previously thought [4]. By the end of this period, oxygen levels rose sufficiently to sustain oxygen-based life. These life forms prospered until about 541 mya, when they became extinct due to unknown environmental and biosphere factors.
It was around this time, 540 mya, that an explosion of new lifeforms was seen. This was referred to as the Cambrian explosion. New features including spikes developed. This was a shift towards metabolically active animals with defined head and tail structures for directional movements to chase prey. The organisms expanded their feeding styles to accommodate diversification. Trilobites are one such famous animal of this period. During this time, Stromatolite reef-building bacteria declined and organisms called brachiopods continued to form reefs. The present-day, reef-building organism, corals were formed around 200 mya after the Cambrian explosion.
Anomalocaris canadensis. Image source: https://www.britannica.com/science/Cambrian-explosion
Trilobite fossils. Image source: https://blogs.scientificamerican.com/laelaps/trilobites-had-guts/
Geological Time Scale. Image source: https://www.nps.gov/subjects/geology/time-scale.htm
It was during the Cambrian explosion when most divisions (in plants) and phyla (in animals) were established. Food webs began emerging forming the foundation for Earth’s ecosystems. In the Age of Fishes, i.e the Devonian and Mississippian periods, fishes and corals ruled the oceans. The first forests also began to form. The Age of Amphibians occurred during the Pennsylvanian and Permian periods, where animals began to adjust to terrestrial ecosystems.
Throughout the Paleozoic era (from the Cambrian to Permian periods), there were several mass extinctions, where over 70% of all life forms were wiped off because of many external factors such as environmental phenomena, the emergence of predatory lifeforms, adjustment to the rise and fall of sea levels and tectonic plates activity. Every mass extinction has led to the emergence of new and diverse life forms from the existing organisms because there was more space to occupy and more resources available.
The extinction after the Permian period is called the PT extinction (P for Permian and T for Triassic). This extinction wiped out around 90% of all life forms. This extinction marks the end of the Paleozoic era and the start of the Mesozoic Era.
The Mesozoic Era came to be known as the Age of Reptiles because dinosaurs ruled the planet during this period. The small and big reptiles diverged enormously. Even today, classifying reptiles based on their morphological features is exhausting because of their high diversity and uniqueness. This is largely attributed to the expansion during this era. The first mammals appeared in the Cretaceous period and eventually, the placental mammals also emerged.
After the Cretaceous period, around 66 mya, CP extinction occurred (C for Cretaceous and P for Paleogene). This is characterized by the asteroid hit, which causes the extinction of almost all terrestrial organisms. Only exothermic animals like tortoises and crocodilians, non-avian dinosaurs (birds) rodent-sized and dog-sized tetrapods survived these events. These rodent-sized and dog-sized animals eventually led to the formation of today’s mammals.
Image source: https://www.earthlife.net/mammals/evolution-2.html
Mammals diverged vastly and began looking more like those of the present day. From bats to whales, almost all mammals evolved during the Paleogene period. The climate began to develop into that of the present day. Modern ecosystems like mountain ranges, grasslands, tundra regions, plateau regions, etc. began to form animals moved to these regions to become more and more diverse. Most of the cross-pollination and mutualistic relationships were established during this era. The Homo genus formed around 2.5 mya, after which the Ice-Age and glacial outbursts occurred. The extinction of large mammals and birds began when the environmental conditions changed drastically also because of hunting by modern animals.
As for the present day, all living organisms, are in the midst of another mass extinction called the Holocene extinction. It started around 10,000 years ago, just as the last ice age ended. Humans are accelerating the extinction rate by poaching, deforestation, fossil fuel combustion etc., all of which has caused climate change and the associated melting of ice-caps and rise in sea levels. The current rate of extinction of species is estimated to be 100 to 1000 times higher than natural extinction rates.
Scientists study the past to understand the patterns of climate change that drive the evolution of Earth and its inhabitants so that any future extinction can be predicted. So, along with discovering new technologies, it is important to understand the past. Often, people correlate fossils to dinosaurs but the Earth’s history also consists of many diverse organisms.
Fossils are the lens through which the past is examined. Fossils give an insight into paleoclimates, changes in geographical features, the evolution of living organisms, etc. Understanding the rich history of living organisms gives a sense of appreciation of evolution and hopefully will impact people enough to preserve the current species before they too become extinct.
References
[1] Smithsonian, N.M.N.H. Early life on earth - animal origins [Online]. Available at: https://naturalhistory.si.edu/education/teaching-resources/life-science/early-life-earth-animal-origins/ (Accessed: 27 December 2021)
[2] Mcgowan, A. (2017) Oldest evidence of life on earth fund in australia [Online]. Available at: https://economictimes.indiatimes.com/news/science/oldest-evidence-of-life-on-earth-found-in-australia/articleshow/61658155.cms/ (Accessed: 27 December 2021)
[3] Martin, W.F., Garg, S. and Zimorski, V., 2015. Endosymbiotic theories for eukaryote origin. Philosophical Transactions of the Royal Society B: Biological Sciences, 370 (1678), p.20140330.
[4] Windley, B. F. (2019) Ediacara Fauna [Online]. Available at: https://www.britannica.com/science/Ediacara-fauna/ (Accessed: 27 December 2021)
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