Opabinia
Anomalocaris
For eons, although Earth’s oceans teemed with life, you would not have been able to see a single individual organism. That is because for billions of years the only living things consisted of no more than a single cell.
It is possible that some cells amassed into colonies, but the first fossil evidence of these appears only about 600 million years ago. These colonies were probably rather like sponges, the most ‘primitive’ animal still living, found in many parts of the oceans. In sponges, each cell can live independently, but can also function together with other cells. If a living sponge is broken up into fragments they will in time come together to form a new colony. Sponges are fixed to rocks, and feed on minute particles in the water. There is some coordination between cells, but no real nervous system.
By 590 million years ago a whole range of higher animals had emerged, with more clearly defined body plans and identifiable nervous systems. These creatures were still confined to the oceans, and included coelenterates (such as jellyfish and sea anemones), annelid worms and arthropods. Key characteristics of arthropods include bilateral symmetry, segmented bodies, multiple legs, eyes, and an exoskeleton – a hard external layer that protects the internal organs. All of today’s arthropods – including crustaceans, spiders, scorpions and insects – are descended from some of these early creatures. Other arthropod groups, such as the trilobites, are long extinct.
The trilobites appeared during the Cambrian period (542–488 million years ago), which saw an ‘explosion’ of new animal types, including most invertebrate groups known today. Various possible explanations for the Cambrian explosion have been put forward. The increase in photosynthesizing organisms had generated more oxygen in the atmosphere. By this time the ozone layer had formed, protecting life on Earth from the Sun’s ultraviolet rays. There was also a sudden increase in the oceans of calcium (a key component of hard body parts, such as exoskeletons) due to greater volcanic activity along the mid-ocean ridges. Ecological and evolutionary explanations suggest that an accelerated arms race between predator and prey, perhaps initiated by the development of the first primitive eyes, may have allowed prey to be detected by a predator at a distance – and vice versa.
Prototypes
During the Cambrian explosion a number of groups appeared that resemble no animals we know today. Among the bizarre forms found in the Burgess Shale of Canada was Opabinia , a predator with five eyes and a snout like a vacuum cleaner. Another predator was Anomalocaris , 60 centimetres long, propelled by lateral flaps, and with a mouth that resembled a slice of pineapple. Many of the Burgess designs were only brief Cambrian successes – but Pikaia , an eel-like creature about 4 centimetres long, has been interpreted as an ancestor of the vertebrates, members of the phylum Chordata, which includes humans.
Opabinia
Anomalocaris
The next key event was the appearance of the first true vertebrates. Vertebrates are creatures with internal skeletons, including a spine made up of linked vertebrae protecting a spinal cord – the central component of more advanced nervous systems. A few obscure creatures living today have a spinal cord but not a spine, and it is thought that their ancestors first emerged in the Cambrian. But the first true vertebrates, the jawless fishes (similar to modern lampreys), appeared around 500 million years ago.
In these early fishes, the internal skeleton was made of cartilage, not bone. This is still the case with a number of fishes today, including sharks and rays, whose ancestors were the first to develop jaws, around 410 million years ago. Jaws are a characteristic of nearly all vertebrate groups today.