The earliest life forms extend around 3.8 billion years ago. Throughout the earth’s life-history, organisms have become increasingly diverse and complex. The fossil record offers evidence of different life forms in different periods; however, it has gaps and limitations.
We know from the fossil record there is a ‘background extinction’ rate in which 10-20% of species per million years disappear. In mass extinctions, more than 50-70% of all living species die out.
Figure 1 illustrates the 5 major mass extinctions, often referred to as “the big five”.
Two defining features of the mass extinctions are that many species go extinct within a relatively short time frame (caused by one or a cluster of causes) and that the extinction affects a wide range of habitats and species. The impact of the extinction event is always that there is a dramatic change in the organisms which come to dominate immediately after the extinction event.
The remains or traces of organisms in rocks are called fossils. Since these fossils are embedded within the rocks, the latter can be dated, and this gives us an indication of the geological age of the fossil. Signs of early life are submicroscopic and difficult to find, requiring sophisticated equipment and processes such as chemical experiments, electron microscopes, x-ray analysis, Raman spectroscopy and mass spectroscopy.
Hypothesis for life on earth:
· Oceans
· Meteorites (amino acids coded for by DNA; DNA allows for Darwinian evolution)
The earliest life on earth was probably microbial archaea, which are extremophiles. Owing to their resilient survival capabilities, microbes may be the most successful organisms on our planet.
Conventionally, complex multicellular forms were thought to have appeared about 575 million years ago. That is billions of years after the emergence of microbial life. However, recent discoveries in West Africa, showed highly complex multicellular forms that date to about 2.1 billion years. From this you can see that fossil record timelines are very dependent on what fossils have been discovered and analysed.
542 million years ago there occurred the Cambrian Explosion, when scientists find a wide variety of multicellular organisms in the fossil record. The pace of diversification after Cambrian was extremely rapid. Within 5 to 10 million years of the Cambrian explosion we have all relatives of modern phyla. We’re not sure why this incredible diversity emerged: some hypothesise that higher oxygen levels enabled it, others suggest that the presence of homeotic genes, which regulate the development of anatomical structures, played a key role in this diversification. The atmosphere of earth changed 3.7 billion years ago, when aggregates called “stromatolites” (the earliest fossil evidence of life on earth) formed of cyanobacteria began to photosynthesise and produce oxygen.
An Abundance of Bacteria:
Life is divided into 3 domains.
Microbes have been very successful, they are the origin of life.
Lynn Margulis work shows us that at some point, 2 or more billion years ago, a bacterium ended up inside an archaea and became the origin of the nucleus. The bacterium (which produces energy) ended up inside an archaeal cell and gave it an energy source, allowing it to become more complex and the origin of all eukaryotes.
After that, a photosynthetic bacterium ended up inside another organism which provides the origin of photosynthesis.
Cyanobacteria are the beginning of the photosynthetic group and when they evolved they were an important step in establishing conditions of life on earth. They call it the great oxidation event. Early earth lacked oxygen until it was created as a by-product of photosynthesis from Cyanobacteria. Oxygen dissolved in water and oxidised with Iron to form distinct bands in marine rock.
We have a mutual symbiosis with bacteria, they are roughly equal to our own body cells.
Panspermia is the hypothesis that life exists throughout the universe and can be distributed by space dust, meteoroids and asteroids, comets, planetoids and spacecraft carrying unintended contamination by microorganisms.
Unicellular organisms can often group together to form colonies. Multicellular organisms can consist of anywhere from a few cells to billions of cells, which are organised to form tissues, organs and organ systems. The overall functioning of such multicellular organisms relies on the complex coordination of the different specialised systems.
The term radiation (or adaptive radiation) refers to the proliferation of species over a relatively short period of geological time. One of the most spectacular adaptive radiations is the one popularly known as the Cambrian Explosion that occurred about 542 million years ago.
Extinctions refer to the dying out of species without any descendants. After each extinction event, there appears to have been a distinct adaptive radiation of organisms to occupy the vacated habitats. A classic example of this is the prolific radiation of mammals after the extinction of the dinosaurs at the end of the Cretaceous.
Related Links:
https://www.bbc.co.uk/news/science-environment-39117523
https://theconversation.com/how-we-discovered-the-worlds-oldest-fossils-73802
https://www.scientificamerican.com/article/experts-where-did-viruses-come-fr/
https://www.frontiersin.org/articles/10.3389/fmicb.2014.00742/full
https://www.sciencedaily.com/releases/2011/01/110125172439.htm
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