Past, Present and Future: the Marvels of Evolution
Life appeared on earth around 3.8 billion years ago. The first living organisms were simple bacteria, which developed in water. Over the course of time, these organisms multiplied and diversified. Some left the oceans for the terra firma. Some even managed to become airborne. Today, millions of species and billions of individuals populate our planet. These species have undergone numerous changes over the course of their evolution. Some have occurred slowly over several generations. Other more rapid changes are related to mass extinctions such as that which resulted in the end of the dinosaurs (with the exception of birds).Let’s have a closer look at some key moments of evolution...
The strange animals of the Cambrian period
There are few traces of organisms that lived before this period as most of them had soft bodies and rarely fossilized. In the Cambrian period around 540 to 490 million years ago, animals emerged with some of their body parts mineralised. Exoskeletons, shells, spines, plates and pincers, these hard body parts were forms of protection for prey and weapons for predators. They also helped to support the animal and enabled muscles to attach.
As the Burgess Shale (Canada) shows, Cambrian fauna diversified rapidly and extensively (giving rise to the expression Cambrian explosion). Dating to around 505 million years ago, the fossils at this site are so well preserved that even the soft parts of some can be distinguished, which is extraordinary.
Here: Marella splendens (up to 20mm in length)
With five eyes, 24 feet, rows of spines, or an armour of small plates, these strange fossil creatures discovered in the Burgess Shale certainly capture the imagination. Our model-maker worked for five months to meticulously recreate of five of these fantastic animals.
Here: Aysheaia pedunculata (1-6cm long)
The most common species to be found in the Burgess shale is Marrella splendens, with 15,000 recorded fossils. It was a small marine arthropod that barely reached 20mm in length. It is recognized by its head shield topped by four backwards-pointing spikes.
Hallucigenia sparsa is probably one of the strangest and most difficult to determine of the Burgess specimens. This animal’s body is long and soft. It has seven pairs of spines and several tentacles (large ones at what is believed to be the back and small ones at the front). They can measure up to 3 cm in length.
The trilobites are the most diversified category of fossilized marine arthropods. There are more than 18,000 identified species. These different species occupied vast areas and succeeded one another in quick succession. This is what makes them good fossil date indicators. If two geographically distant geological strata contain the same species of trilobite, we can conclude that these strata date from the same period.
This trilobite, probably a Gerospina schachti, is small (38mm long), but some species measure up to 70 cm.
In the expansive waters of the Devonian period
A huge ocean covered most of the planet during this period (between 416 and 359 million years ago). The shallow and relatively warm seas bordering the continental masses were populated by trilobites, crustacean, gastropods and jellyfish. The armoured and jawless fish of the beginning of the Devonian period were gradually replaced by jawed fish which were no longer satisfied with soft food or plankton and became active hunters.
Dunkleosteus was one of the largest placoderms ever (actual size of the skull: about 110cm long, 60cm high, 60cm high).
The placoderms were the first jawed fish. Powerful, they did not have teeth but instead sharp, bony plates.
As you can see with this Bothriolepis canadensis, the front of their body is covered with an armour of bony plates. The last of this species disappeared at the end of the Devonian period without descendants.
Actual size: 16.5cm high
Dating from the end of the Devonian period, Acanthostega gunnari is a primitive tetrapod living in an aquatic environment. Its limbs look more like legs than fins, but its joints, too stiff to bend, will not yet support the body outside of the water.
Actual size: 11.5 cm in length
In the forests of the Carboniferous period
Before the Devonian period, the shores were invaded by tiny plants. After the appearance of spores and then seeds, they
colonised the hinterland. Thanks to the emergence of leaves, needles, wood and roots at the end of the Devonian period, they became bigger and bigger. In the Carboniferous period (from 359 to 299 million years ago), the climate was warm and humid, at the beginning at least, and there were no distinctive seasons, which fostered the increase in biodiversity. Fertile marshland and forests were home to many insects, spiders, scorpions, eurypterids, millipedes, snails and slugs as well as the first terrestrial tetrapods.
Giant lycopods, horsetail, tree fern and conifers are the plants to which the Carboniferous period owes its name. Instead of being decomposed by bacteria, funguses and insects, as would happen today, all of these plants accumulated and formed coal over the course of time. The imprint of the stem of the lycopod Sigillaria ovata.
Actual size: 60 cm in length
The temnospondyls were amongst the first tetrapods to escape the aquatic environment. But these giant amphibians were probably still heavily dependent on it as they could only lay their eggs there. Most have a large, wide and flat head. But that of the Archegosaurus decheni is just 28cm and ends in a long, thin snout which suggests it fed on small fish.
The oceans of the Carboniferous period were teeming with sharks, starfish, gastropods, sea urchins, and cephalopods with external coiled shells (such as goniatites and nautiloids). Placoderms and most of the sarcopterygians (those “lobe-finned” fish gave rise to the tetrapods: amphibians, reptiles, mammals, dinosaurs, birds etc.) had disappeared, leaving actinopterygians (the “ray-finned” fish) like this 30 cm Benedenius deneensis to take their place.
In the seas of the Jurassic period
At the end of the Permian period, the emerged land formed a single block, the Pangea surrounded by a single ocean, the Panthalassic Ocean. During the Jurassic period (from 203 to 135 million years ago), the Pangea fragmented, giving birth to the Atlantic Ocean. Various shallow and warm seas also appeared. They were populated by animals such as corals, bivalves, belemnites, ammonites, crustaceans, fish, sharks, plesiosaurus and ichtyosaurus.
The crinoidea, or sea lilies, were echinoderms like starfish and sea urchins. They were made up of a calcareous stalk at the end of which were flexible arms, which caught algae, unicellular organisms, small crustaceans and the larvae of invertebrates floating in the water. In abundance, fossilized species, such as the Seirocrinus subangularis, formed “meadows” on the seabeds. Having appeared in the Cambrian period, they were badly affected by the most significant mass extinction that earth has ever seen. Almost 95% of marine species and 70% of terrestrial species disappeared at the end of the Permian period, 250 million years ago.
Actual size: around 3 x 2m
With their slender, dolphin-like body, their four legs shaped like paddles and their fin-like tails, the ichthyosaurus, like this Stenopterygius longifrons, were amongst the reptiles best adapted to the marine environment. Along with the plesiosaurus, they were the biggest predators in the Jurassic seas.
Actual size of the skull: 40 cm in length
The diversification of mammals in the Eocene period
The first mammals appeared at the end of the Triassic period, almost 225 million years ago, at the same time as dinosaurs. They survived the mass extinction at the end of the Cretaceous period, 65 million years ago, and continue to evolve. The first modern mammals emerged during the Eocene period (from 55 to 34 million years ago). The earth warmed up at the beginning of this period and was around 10° C hotter than the current average temperature. As the Messel fauna illustrate, primitive species were gradually replaced by more modern and competitive ones.
There is an exceptional fossiliferous deposit at the site of Messel, near Frankfurt in Germany. Dating to 47 million years ago, it owes its reputation to the quality, richness and diversity of its fossilized fauna - crocodiles, snakes, lizards, frogs, turtles, birds and insects as well as numerous mammals. This site provides an excellent illustration of the transition between primitive and modern fauna. For example, the snakes and lizards are rather primitive while some mammals (in particular bats) possess modern characteristics.
The bats from Messel strongly resembled their modern-day cousins, even though they probably did not yet possess echolocation. Here: Palaeochiropteryx sp. (actual size: 7cm tall)
This Kopidodon macrognathus was a small arboreal (tree-living) herbivore, now extinct.
Like today’s squirrels, it had a long, bushy tail serving as a pole for its balance as it leapt from branch to branch.
Larger specimens reached 115cm; this one is a little over 70cm long.
47 million years ago, the Messel site was a tropical lake. It is no surprise that fish, fresh water turtles, salamanders, frogs (such as this Eopelobates wagneri, 8cm high) and even crocodiles were discovered there.
Amongst the mammals found at Messel is this magnificent Eurohippus messelensis. Related to modern horses, it was just over 50 cm long, had four digits on each foreleg and three on each hind leg (as opposed to the single hoof of the modern horse), and lived in forests.
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Evolution today
Man is playing a significant role in the current evolution of species by crossing and artificially selecting them and producing them in laboratories by manipulating genetic information.
Man also exerts significant selective pressure over some populations. For example, the nets used in commercial fishing retain large cod, giving small-sized adults more chance to escape and reproduce. As a result the small-size genes are more frequently passed on and the average size of the adults is decreasing.
But more importantly, man is contributing to the disappearance of various species today, either directly or indirectly, through deforestation, the destruction and fragmentation of habitats, excessive urbanisation, overuse of natural resources, pollution, climate change etc.
Here: the endangered Siberian tiger.
The thylacine, also known as the Tasmanian wolf or Tasmanian tiger, is an Australian marsupial. Or at least it was: Benjamin - the last one in captivity - died in Tasmania’s Hobart Zoo on 7th September 1936.
This animal was a victim of prejudice and ignorance about its way of life. The carnivore hunted at dusk and could open its mouth very wide, making people suspect that it was a threat to sheep and so they eliminated it systematically, encouraged by bounties. Things could have been different: thylacines were easy to tame.
New varieties of roses, orchids and tulips. Botanists are expert in the mechanisms of artificial selection. They systematically choose the specimens with the required characteristics (colour, odour and resistance to frost), and cross them again and again until the new variety exhibits these characteristics from generation to generation.
In the animal kingdom, breeders can attempt to foster creatures that run the fastest, carry the heaviest loads or produce the most milk. With the “Belgian Blue Beef” breed of cows (photo), the key consideration is muscular development and therefore the quantity of meat. This breed of cow today makes up almost 45% of the national herd, but without man they would not survive. Owing to their morphology – which has been fostered by breeders – most of the cows do not successfully calve naturally, and require a Caesarean to be carried out.
What will the future bring?
The continents are in constant motion. Australia is moving back towards Indonesia and Europe and Africa continue to get closer to one another and will eventually join. A huge ice cap is likely to cover the northern hemisphere making the planet’s climate dry. Fauna and flora will of course adapt to these new conditions, but what will animals look like in 50 million years time? No one knows, but we can at least dream. The scientists who ‘created’ these futuristic animals based them on evolutionary models. Evolution repeats itself continuously, so by observing the past, they can try to imagine what life might look like in the distant future.
Propellonectes russeli, the imagined descendent of the Northern Giant Petrel (Macronectes halli), an existing seabird, has atrophied wings unfit for flight, but disproportionately large feet and an even more hydrodynamic body shape than its ancestor: it is an excellent swimmer. Length: 1 m
With a more massive head and shoulder girdle and hypertrophied, prominent incisors, this Corticochaeris gouldi could be a descendent of the capybara (Hydrochoerus hydrochaeris), the largest living rodent today. Length: up to 2 m
A potential descendent of the common brushtail possum (Trichosurus vulpecula), an arboreal marsupial, this Trichopteryx dixoni has a longer and more flexible prehensile tail, larger skin folds between its feet, and a ventral pouch that opens downwards, towards the tail. Length: 1.5 m, including the tail
