The most recent eon of the Earth’s history, the Phanerozoic, begins with the Cambrian period and it is preceded by a short period that predicted the huge changes that would occur during this epoch – the Vendian.
Oxygen 1 – Carbon Dioxide 0
At the beginning of the Phanerozoic the quantity of oxygen would have increased again, this time up to levels that were already close to the actual. It is not known what drove to this occurrence, but it’s thought that the development of the sulphate-reductor bacteria would have led to the accumulation of larger quantities of species produced by the photo-synthetizers (like sulphur and sulphurets), which left behind an excess of oxygen in the air. Again, the erosion caused by the release of acid substances into the air (resulting from reactions between the oxygen and other elements) and the subsequent release and dispersion of nutrients would have favoured the growth of the number of living organisms.
The proportion of carbon dioxide in the atmosphere declined again and, more slowly, this evolution still prevails, encouraged by the increase of the solar temperature and the need for keeping the thermal equilibrium that is felt by the organisms on Earth. The combined action of the organisms provokes, this way, the gradual reduction of the presence of carbon dioxide (greenhouse gas) in the atmosphere.
During the Vendian (between 650 million and 540 million years ago), a completely new variety of soft-bodied organisms with macroscopic dimensions appeared on the Earth’s surface. Its appearance may have been encouraged by a mass extinction of previous organisms like the stromatolites, which may have been caused by the fall of a meteorite on Earth. The continuous increase of the atmospheric oxygen would have given birth to the appearance of an ozone layer. Because this ozone layer protected the biota from the damaging effects of the ultraviolet radiation, it shall have also contributed for the realization of this biological evolution.
The emergence of multicellular organisms had its origin in independent cells that came together and formed cells’ colonies. Inside them, they started to perform different and complementary functions, as it happens today in the case of the porifera (sponges). It was a symbiotic mechanism, similar to the one that made the eucaryots appear, that motivated the emergence of the multicellular living beings. About 540 million years ago, this primitive fauna became extinct, which is a fact that could also have been caused by a meteoritic event.
The Salinella, one of the strange organisms that emerged during the Vendian (Kerry B. Clark)
Explosion of Marine Life
During the Cambrian, an explosive emergence of a vast variety of organisms provoked the appearance of most of the direct ancestors of the great animal groups (phyla) that we know today, as the arthropods endowed with external skeletons (which included the trilobites and nowadays include the insects, arachnids, crustaceans, etc), the mollusks (ancestors of the octopuses, snails, oysters, etc), the echinoderms (ancestors of animals like the modern sea stars or the sea urchins and, through a parallel lineage, the vertebrates), the brachiopods (with few actual representatives), among many others.
The growth of the quantity of atmospheric oxygen may have allowed the emergence of organisms with big dimensions, because it enabled the development of respiratory and circulatory systems, opposite to the skin respiration previously dominant that is efficient only in small animals. The climate would be hotter during this epoch, given the absence of continents at the poles that could support the accumulation of glaciers (which, because they are white, reflect the light back to space and cool the Earth) and the blockage of the tropical waters by continents that prevented them from reaching the poles and cool down.
Just like the life in the Vendian, the life of the Cambian also seems to have crossed a phase of mass extinction, having been replaced by the life of the Ordovician.
Life in the Cambrian (University of Michigan)
During the Ordovician (which began about 505 million years ago) emerged the first vertebrates (as the several classes of fish) and were developed the cephalopod mollusks (which today include the octopuses, the squids and the nautiluses) that, along with the vertebrates, were the only group of animals to reach a high rank of intelligence. Across the eras, the development of the intelligence was gradually endorsed through the natural selection mechanism – the one that determines that the organisms that are more able to dominate and modify the environment have higher chances of surviving, when they compete with other organisms.
Life in the Ordovician (University of Michigan)
During the following period, the Silurian (which began 440 million years ago), there would have emerged the first land plants, which were still unprovided with leaves. They would be equipped with primitive water-pumping systems (the ancestors of the roots) and would thrive mainly at the shores of the bogs.
The Cooksonia, a plant of the Silurian that inhabited the shores of the bogs (Adrian Sington)
However, only during the Devonian (which began 410 million years ago) the land plants expanded in a more remarkable way, having increased the proportion of occupied lands and grown in size and variety. It’s thought that the ferns were particularly abundant.
With the aim of exploiting this new and untouched food source, they also appeared the first land animals. The arthropods, including unwinged insects and, more remarkably, the myriapods (millipedes and centipedes), were the pioneers of this exploitation, having some of them reached enormous sizes (up to 2 metres long) due to the absence of predators. Nowadays, the land arthropods are small due to their heavy external skeletons, being preferable to become nearly undetectable in front of their lighter predators.
Little after the arthropods, they emerged the first land vertebrates – the amphibians. This new class of animals developed from fish that presumably had acquired the capability for breathing the oxygen of the atmosphere (lunged fish), which gradually transformed their fins into legs.
Life in the Devonian (R. Scorpion & C.R. DeCorse)
While life spreads on land, in the sea there thrive primitive fish species, like the armoured fish or the cartilaginous fish (ancestors of the actual sharks and rays), in opposition to the bony fish, which appeared later and are dominant today.
During the Carboniferous period (which began 360 million years ago), vast extensions of land were covered by dense forests of plants representing groups that are minoritary today, like the already mentioned ferns.
Some land vertebrates began reproducing themselves through amniotic eggs, which prevented that the animals became dry in their interiors. These animals, the reptiles, started to put eggs on land, overcoming the amphibians in what concerns to their capability for adapting to a non-aquatic environment and, therefore, they achieved to expand into the interior of the continents.
A carboniferous forest. Above, to the left: a meganeura, the largest insect that ever existed (Adrian Sington)
Above: the Eogyrinus, an amphibian from the Carboniferous period; below: the Hylominus, the most ancient reptile known, which lived during the Carboniferous period (Adrian Sington)
The Permian period began 285 million years ago and was characterized by the dryness of the interior of the super-continent that had been formed then – the Pangæa. The reptiles with an appearance similar to the mammals (mammal-like reptiles) thrived during this period. They typically had a bone structure and a dentition similar to the mammals’ and sometimes they had their bodies covered with hair.
A Permian scenario: In the foreground, two mammal-like reptiles (Adrian Sington)
The Era of the Dinosaurs
At the end of the Permian period (245 million years ago) it occurred the most catastrophic mass extinction that is known, caused by reasons that aren’t yet known for sure. It was probably caused by disasters (large scale vulcanism, release of sulphur from the mantle, acid rain) triggered by a collision with an asteroid or a comet with a size between 6 and 12 km. A crater in the northwest coast of Australia, with a diameter of about 200 km (larger than the Chicxulub crater in the Gulf of Mexico, related with the extinction of the dinosaurs), may be related with this catastrophe. Less likely this extinction may have also been cause by the impact provoked by a supernova of a close star. High traces of iridium found in rocks at the Permian-Triassic boundary layer favour the first hypothesis. After the occurrence of this catastrophic event, the forests of ferns (which reproduce themselves by spores) gave place to gymnospermic plants’ forests (plants that reproduce themselves by seeds), and the Permian reptiles gave place to the dinosaurs (which were also reptiles). The true mammals (similar to small shrews), descendant of the mammal-like reptiles, appeared for the first time during this period. They arose also the earlier flying vertebrates – the pterosaurs (reptiles) – which may have developed from arboreal reptiles that soared between the trees. Some reptiles, like the ichthyosaurs, the pleisiosaurs and the giant liopleurodons, followed the inverse way and went back to the water, just like the ancestors of mammals like the whales or the dolphins did in the past or like the walruses, the seals or the sea lions are doing today.
A Scene of the Triassic: To the left, the fearful Postosuchus (a reptile that existed before the dinosaurs); to the right, two Placerias (mammal-like reptiles) (Martin Hendry)
The Peteinosaurus, one of the earliest pterosaurs of the Triassic (Martin Hendry)
It was during the Jurassic period (which began 208 million years ago) that the descendants of the first birds started to thrive, having with the scales already transformed into feathers but still endowed with teeth at their beaks, with a reptile-like bone structure and claws at the tips of their wings. Later, they surrendered the pterosaurs as the dominating species of the skies. It’s believed that the birds are more ancient than the dinosaurs and they shall descend from warm-blood reptiles, so that their feathers could become an advantage for the animals – the feathers helped them to keep their internal.
Jurassic landscape (Adrian Sington)
Two minoritary groups of the Mesozoic - above: the Archæopteryx (one of the most ancient birds known), from the Jurassic period; below: the Tæniolabis, a mammal of the cretaceous period, representative of a group originated in the Jurassic - the multi-tuberculated mammals (Adrian Sington)
The division of the Pangæa, which began in the Jurassic period, was reinforced during the Cretaceous (which began 146 million years ago) and fomented increasing differences between the fauna and flora of the various regions of the globe. More modern birds and mammals, along with the first flowered plants (namely, the magnolias) appeared or started to thrive during the Cretaceous, being the most primitive ones deprived of colours and smell. The appearance of the flowers encouraged the simultaneous appearance of several groups of insects that depend on them, like the butterflies and the bees.
Magnolias (John Frett)
The Great Impact of Chicxulub
A meteoritic impact that occurred 64,7 million years ago close to the coast of Mexico shall have put an end to the era of the dinosaurs and other previously important animals, like the ammonites and belemnites (cephalopods). Huge fires and sea earthquakes seem to have been immediately caused by this event. A dark cloud of debris thrown by the impact to the atmosphere shall have provoked a period of darkness and global coldness during a decade, a phenomenon that shall have been the final responsible factor for the extinction of the animals.
The impact of Chicxulub: The few hours of fires, land and submarine earthquakes are followed by months of harsh cold and darkness (Ron Miller)
The Era of the Mammals
The extinction of the dinosaurs paved the way for the success of the mammals, which from then on grew in size and occupied the supreme place of the hierarchy of the planet’s animal life. The mammals are divided in 3 main groups: the monotremes (the most primitive), the marsupials and the placentary mammals.
The monotremes, like the reptiles, put eggs. However, they have their bodies covered by hair, they are warm-blood animals (able to regulate their internal temperature, independently of the external temperature) and they breast feed their offspring.
Opposite to the monotremes, the marsupials and the placentary mammals bear active babies. However, the marsupials have very simple placentas, so the baby, when is born, is still at a very premature stage. It climbs to the mother’s pouch and it breast feeds and grows there.
On the other hand, the placentary babies are already born at a quite developed stage. It was during the primordial times of the Terciary period (the period that followed Cretaceous) that the first primates arose, the group of animals to which the humans belong.
An ornithorhynchus, which is a monotreme (Graham Allen)
A newly-born marsupial in the pouch of its mother