There is grandeur in this view of life
THE EVOLUTION OF THE EARTH
The vermillion beams of the sun broke through the black dawn and illuminated a monochrome wasteland, populated with spires and monoliths of stone that extended across the panorama. For how many days had the sun dawned upon this desolate landscape where there was nothing but silence? Time unimaginable. No fish swam in those seas. No philosophers marvelled at their own existence. No astronomers saw themselves in the constellations of the heavens begemmed laquearia.
This was the Precambrian aeon that covered about 4 billion years of earths history and ended 570 million years ago. It covers almost 90% of Earth’s geological history. The Precambrian can be divided into the Archaean, Proterozoic and Priscoan aeons. During this time the Earths crust went through rapid changes. In the Archaean aeon the crust was highly mobile with tectonism and later in the Proterozoic aeon the crust stabilised. At this point geosynclines had formed, which are depressions in the crust. An oxygenated atmosphere also formed. From this derelict wasteland arose a terrestrial paradise wild with crawling matter.
The Hadean period (4.53 Ga) was a world of magma and desolation. The crust comprised gabbros, a plutonic crystalline rock and anorthosite, a granular igneous rock. Vast oceans of dry magma extended across the panorama of the horizons, filled with rock and hot fluid, the blood of the earth that leaked and bled from the boiling liquid veins of the crust. These oceans of molten rock cooled and solidified and the continents were born 190 million years after their solidification. It was a rock landscape bombarded by ice asteroids that brought H20 and other compounds for 200 million years. The bombardment ceased 3.9 Ga (Santos et al. 2017).
The first oceans were a toxic and dismal place, no life swam those poisonous waters. Clean water was eventually produced in inland lakes that were cut off from the sea. Sufficient nutriment such as CO2, H2O and N2 produced the birthplace for prebiotic life.
Image Credit: Ansel Adams
How did the first molecules lead to replicating organisms? Situations such as competition and limited resources would have been a selection pressure on these early replicating structures. Certain forms of replication in the prebiotic seas would have been faster than other forms of replication and thus a form of natural selection would have occurred. By gradual steps, small and insignificant, the grand structures of life appeared. The more complex assemblages that replicated faster were selected over those that replicated slower. Thus primordial life, the ancestors of all, broke forth from soulless rigid matter.
Within the noxious abyss of the aquaria, formless matter became animated. Chemical reactions produced organic compounds such as amino acids, proteins, enzymes and ribozymes etc. A molecular system is alive if it is able to replicate and there needs to be some form of variation in order for evolution to take place. Scientists speculate that the first life evolved with RNA based genomes. Ribozymes are RNA molecules that act as enzymes and they are capable of self-replication. The first organisms were self-replicating RNA molecules that later developed into the DNA organisms. Biologists have attempted to recreate this RNA world in their labs and have been experimenting with ribozymes. Nevertheless, much work is yet to be done (Müller 2006).
Protocaryotic organisms formed in the Archaean aeon, which were first capable of cell division. The oldest organisms were microscopic carbonaceous lifeforms that have been found in 4 billion year old rocks such as cherts and shales. Even today there are organisms that are similar to those ancient lifeforms that occupy hot springs of 100 degrees celsius (Sutton 1974). The microfossils of these lifeforms have been found in rocks in locations such as Ontario. In the later Precambrian times, multicellular Metazoa of higher complexity developed and this was propelled by the heightened oxygen in the atmosphere.
The Cambrian Explosion (540-520 Ma) was propelled by changes to the Earth. Abiotic changes such as oxygenation and changes in sea water composition sped evolution that produced multicellular organisms of high complexity. Changes to the environment include an increase in oxygen in the atmosphere, and increases in phosphorus and potassium which are essential for metabolism. Heightened oxygen would have increased body size and led to new metabolic activities such as predation and burrowing (Zhang et al. 2014).
The greater amounts of oxygen in the Earth’s atmosphere began to produce an ozone layer and this shielded life from solar radiation. Mass extinctions also drove the evolution of life and this may have been propelled by starbursts in the galaxy. Additionally, the genome of life may have been affected by radiogenic elements such as carbonatite magma (Maruyama et al. 2014, pp. 910).
The primordial oceans were enriched with nutrients through erosion and weathering of the landmasses, which was a factor that allowed the Cambrian Explosion. More nutrients were added to the aquatic environments due to uplift of the crust and rifting. The increased nutrients may have also been caused by the collision of mountain belts. Nutrients that were essential for the development of bones included Ca, P and Fe. Many other nutrients such as P, Ca, K, Mg, Fe, Ca, S, Zn, Mo etc. were vital for cellular metabolism (Santosh et al. 2014).
Some scientists have argued that the geotherm in subduction locations of the cooling Earth around 600 Ma may have led to hydrated slabs. The subduction of these slabs introduced large amounts of ocean water into the mantle and thus the sea levels were lowered. This also led to an increase in oxygen in the atmosphere (Maruyama et al. 2014).
The Cambrian Explosion produced metazoans that developed from unicellular lifeforms, which later developed into multicellular creatures during the Phanerozoic aeon (Santosh et al. 2014). Solar energy also benefited life on Earth as a fuel source. Cyanobacterial stromatolites took solar energy as fuel in the Archaean aeon.
Ediacaran ecosystems had an affect on the aquatic ecological conditions. Sponges, which are porous sedentary invertebrates, brought in currents of water to extract nutrients, ventilating the water through sponge pumping. Sponge engineering led to clear water ecosystems in the Phanerozoic oceans (Zhang et al. 2014).
Anomalocarids ruled in the Cambrian seas as the greatest of predators. They were invertebrates from the Cambrian and they are characterised by alien morphologies. They possessed segmented appendages and flexible lobes on each segment. Their mouths was encircled by plates that look like a sliced pineapple. They also had blades that were attached to the dorsal side of the body. Most Anomalocarids occurred in the Middle Cambrian with the oldest species known being Cassubia infercambriensis. Later species also lived in the Early Ordovician and the Lower Devonian (Roy et al. 2011).
Image Credit: Roy et al. 2011
It can be difficult to understand evolution and structure of the brain and optic lobes of Cambrian taxa as soft tissue is rarely preserved in the fossil record. Scientists must interpret functions from the external features and appendages. Sometimes, however, a well preserved specimen does occur and illuminates much about these ancient creatures. An arthropod called Fuxianhuia protensa was discovered from 520 Million years ago from Yunnan Province in China with the brain and optic lobes preserved. Such a sophisticated brain would have led to greater and more complex visual behaviours. The eyes of the Cambrian were much similar to modern insects. Fuxianhuia contains a head shield with a pear of antennae. The thorax is segmented with folds and the abdomen contains several spines. A pair of eyes were located at the end of stalks. This specimen shows that a sophisticated crustacean brain had evolved far earlier than originally thought in the early Cambrian (Ma et al. 2012).
The evolution of the nervous system improved motor coordination and this propelled developmental complexity. The emergent appearance of consciousness involved microtubules that led to more sophisticated behaviour. Associative learning includes processing similarities between different sensory stimuli and responses. Memories of experiences allow associative learning to anticipate the future and rewards. Associative learning and the emergence of experience allowed animals to exploit niches and had a tremendous influence on the development of life on earth (Ginsburg et al. 2010).
Associative learning may have been a factor that propelled the Cambrian Explosion. Changes in memory and learning would have led to a great diversity of new organisms as it allowed lifeforms to work in new niches and this led to new ecosystems. New relations and adaptive responses would have resulted. As organisms became larger, neural organisations in the brain were affected. The evolution of eyes, hard parts, and predation may have been a result of the development of associative learning and this led to the development of consciousness (Ginsburg et al. 2010).
It is a most extraordinary revelation that these small creatures, the ancestors of all, would produce progeny that would go on to compose symphonies and epic poetry, construct rockets, build pyramids and solve the mathematical riddles of the cosmos. They are our ancestors and it was their survival in the primordial seas that led to our existence.
Image Credit: Walcott 1844
The Ordovician Period 485 million years ago occurred after the Cambrian and ended 444 million years ago. The Ordovician saw many changes to the Earth in terms of climate and environment and this sped the diversification of marine taxa. An ice age also occurred that resulted from a cooling earth and less volcanic activity. The cooling of the climate led to heightened ocean circulation and the Earth experienced changes to its tectonic activities (Algeo et al. 2016). A mass extinction had occurred at the end of the Ordovician that killed 86% of invertebrate species.
There have been many other mass extinctions throughout history that have doomed many species. Our ancestors had come close to death innumerable times. The mass extinctions that occurred had disrupted the evolutionary routes that led to our existence. Had not those extinctions occurred, history would have been different.
The Ordovician is notable for being a stage when plants evolved on land as well as the evolution of zooplankton clades and new forms of plankton. The changes in plankton abundance can be linked to changes to the oceans and the increase of O2 in the atmosphere (Algeo et al. 2016).
Image Credit: Walcott 1844
Many organisms from the Cambrian began to diversify in abundance and new organisms also appeared. In this ancient and primitive sea swam creatures such as cephalopods, conodonts, radiolarians, nautiloids, graptolites, eurypterids, arthropods, gastropods, asteroids, brachiopods, trilobites and echinoderms. Endolithic organisms that live or penetrate into stone had evolved in the Late Ordovician such as bivalves (Algeo et al. 2016).
The reign of the anomalocarids was coming to an end. The aquatic ecosystems began to change significantly as the anomalocarids had died out and were replaced by much larger predators. Death, monarch of nature, reigns in tyranny and life is forever replaced. The Ordovician, however, would see many wonderful new forms of life that venture the oceans and take their steps on land.
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EPILOGUE: We are the descendants of microbes that battled for survival in the toxic liquid inferno of the primordial world, a world where they could have been destroyed innumerable times. History is drunk chance.
We are stardust animated. We are microbes on a momentary marble spinning through the vast gulf obscure. You can visualise a time before life where everything and everyone you ever knew or loved was nothing but equal matter unformed and mixed in the Tartarean alembic of interstellar space. There were no voices, dreams or deaths.
What glorious beings arose from this matter! What minds and voices that can dream and meditate! Time escapes with cold indifference and seizes all life into the Night nonexistent. Therefore we must be thankful for our origins as it is the most enlightening of revelations science has illuminated.
Featured image above: Image Credit: NASA/JPL
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