Evolution: First Life

From the first top predator to the first animal to have sex, Paul Chambers introduces pioneering creatures from the very roots of the tree of life.


BBC Wildlife spread
For three billion years, life on Earth consisted of single-celled bacteria and algae. It is not until about 570 million years ago (mya) that we encounter organisms with features that could – just possibly – represent a head, tail, mouth or other recognisably modern organ.
Within a few million years, the earliest, barely discernible animals had evolved into hundreds of new species. This moment is called the ‘Cambrian Explosion’, the results of which are preserved at a handful of exceptional fossil sites.
Cambrian animals may look weird, but amid the strangeness are vital biological innovations: the first eye, the first backbone and so on. It is in this brief geological window that most living animals have their oldest known ancestor.
First true land animal
Name: Arthropleura
Lived: 340–280 mya
Size: 2.6m
Diet: Plant-eater?
The dense Carboniferous forests of 300 million years ago were home to the largest arthropod of all time. Arthropleura was a giant centipede-like creature with thick armour plates and 30 pairs of legs. It belonged to the arthropleurids, an extinct animal group related to centipedes and millipedes, which evolved in the sea but moved onto land about 410 million years ago.
These pioneers – the oldest known terrestrial animals – were plant-eaters; arthropleurids with predatory lifestyles may have emerged later. Arthropleura itself is thought by some people to have been a predator, though most experts now believe it was probably a herbivore.
First apex predator
Name: Anomalocaris
Lived: 535–520 mya 
Size: Up to 2m
Diet: Predator
Anomalocaris was the great white shark of its day, cruising the shallow Cambrian seas in search of prey. It could grow to the length of a modern human, was fast, had good eyesight and possessed a large circular mouth made from razor-sharp plates.
We know from its fossilised faeces that Anomalocaris hunted trilobites and primitive shrimp-like animals. It would probably have held its victims in its two huge front appendages and then crushed them, passing the pieces to its mouth. 
As the first top predator, Anomalocaris may have been responsible for an early evolutionary ‘arms race’, forcing other animals to develop hard shells for protection.
First to have a head and a tail
Name: Spriggina
Lived: 550 mya
Size: 4cm
Diet: Not known
Discovered in the Ediacara Hills of Australia, Spriggina fossils show what are probably the oldest examples of a ‘head’ and a tail. Many earlier animals had a circular profile resembling modern anemones and jellyfish, but the body of Spriggina was covered in rugged, plate-like structures.
The creature has variously been described as a primitive worm, a sort of sea anemone and the ancestor of the trilobites.
Some palaeontologists think that they have found evidence of spines on its head, and believe that it may have been the oldest known predator. Others argue that it couldn’t move at all or that it crawled slowly across the seabed. Much about Spriggina remains a mystery.
First spiny animal
Name: Hallucigenia
Lived: 535–520 mya 
Size: 3cm
Diet: Detritus
Hallucigenia’s dream-like name reflects the fact that its fossils were unlike anything seen before. They suggested that it was a small, worm-like animal with no obvious head, and a body covered in spines and tentacles.
The first reconstruction showed the animal walking on its spines, but nowadays it is usually drawn the other way up, with the tentacles as feet and the spines acting as protection against predators such as Anomalocaris (see above).
True to its name, Hallucigenia remains an enigma: its lifestyle, biology and evolutionary status are all uncertain, though it could be an ancestor of today’s tropical velvet worms.
First to move
Name: Kimberella
Lived: 560–550 mya 
Size: 15cm
Diet: Algae
Kimberella was thought to have been a type of jellyfish until specimens from Russia revealed it as a slug-like creature that grazed algae on the seabed. Fossils of its feeding tracks suggest that Kimberella was the first animal to move in search of food.
This was a major advance, since other species attached themselves to the seabed or floated with the current. Similarities between Kimberella and living molluscs could mean that it is the ancestor of many modern animal groups, including mammals.
First to have an eye
Name: Trilobite
Lived: 520–248 mya
Size: 0.5–80cm
Diet: Scavenger
The trilobites were one of the most abundant animals in ancient seas – more than 15,000 species are known from fossils. Besides being the oldest arthropods, they also possessed the first eyes. These were made from dozens of small crystals (a bit like those of modern insects) and were a major evolutionary innovation.
Eyes – and a tough exoskeleton – gave trilobites some protection, but they were still eaten by many predators, including Anomalocaris (see above) and Pterygotus (see below).
Most trilobites were scavengers that burrowed through soft sediment, eating food particles and small animals. The evolution of large predatory fish, such as sharks, probably contributed to their extinction.


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