560-540 MILLION YEARS AGO

Around 555 million years ago, worm-like creatures called Ikaria wariootia, 7mm long and 2.5mm wide, emerged from the shallow sea beds to feed on organic matter. What makes this species rather interesting to the scientists is the discovery that it is the first and earliest known bilaterian, meaning it has two symmetrical sides to its body with openings at either end connected by a gut. Furthermore, the creature appeared to have distinctive tail and head ends and so giving scientists considerable confidence that this creature probably had a proper mouth that looked different from the exit region of the gut known as the anus. Researchers at the University of California Riverside believe this creature was the among the earliest bilateran animals. But whether this creature is the ancestor of most animals that exist today, including humans, is not yet known.

Among the animals that existed and lived near the surface of the water included Yorgia, Dickinsonia, Tribrachidium, Charniodiscus and Kimberella. ranging in size from 3 centimetres to up to 9 inches long. There will be some simple proto-mollusks and slugs carving out an existence. But giving their pre-disposition to moving slowly, it would not be surprising they developed mechanisms to protect themselves, such as toxins or poisons to hunt and defend with. There were predators and prey at this stage but we have no idea how their actual biology worked.

Plant life of the variety we might be familiar with today on land were non-existent. Not even the first trees would appear until about 370 million years ago. Instead, we have mats of algae living in huge slimy colonies on the surface or the floor of the shallow waters and on wet rocks. No early cellulose plants (technically wood). This was a time when photosynthesizing algae and cyanobacteria were the only prolific non-animal-based organisms on the planet.

The ozone layer was at a level to shield animals from deadly levels of UV radiation at this time, and has been that way since 600 million years ago. had existed since 600 million years ago. However, without enough shady plants, the risk of sunburn from too much UV exposure and mutations accelerating the ageing process and ultimately premature death could be the risk, which is why many animals stayed in the water only to emerge underneath rocks, or come out at night.

Oxygen levels in the atmosphere were rising slowly, but below modern levels. The best estimate is between 10 to 13 per cent. However, carbon dioxide (CO₂) was thought to be about 0.5 per cent (or 4,610 to 4,640ppm), causing the planet to experience a hothouse environment. Again living in the watery environment kept temperatures cooler and more protected from the harsh elements above the water.

But like being caught between a rock and a hard place, the animals were restless. On the one hand, creatures were seeking solutions to the growing numbers of slow moving predators moving around on the floor. But on the other hand, we see for a number of animals seeking refuge near or above the water, the UV rays would impose its own barrier, leading to increases in the number of genetic mutations. Many animals that could survive for a while with new mutations faced uncertain futures unless the mutation could somehow bring substantial benefits to give them the survival advantage. Otherwise, in a "dog eat dog" world, many of the failed genetic experiments among ambitious creatures leading the charge to the watery surface would end up either getting eaten, or die sooner than expected. But through all these genetic experiments, some creatures would develop new physical adaptations to better handle the harsh environment and presence of preadators.

Eventually, the scene was set for another major development in the evolution of life on Earth.

It happened fairly quickly, perhaps in a matter of a million years or so. Short in geological terms, but enough for another explosion of mostly invertebrate multi-cellular animal life forms of enormous variety in the oceans, inland seas, and various lakes peppered over the desert-like expanses took place close to around 542 million years ago. This led to a bewildering array of creatures with different and unusual defence mechanisms, more sensitive eyes to pick out greater details in the shallow seas and sometimes a multitude of these eyes to help give an accurate sense of depth and distance from other animals, and better appendages and body shapes for swimming through the oceans.

Why the sudden explosion in animal diversity? Scientists are not entirely sure, but one thing is certain: the development of specialised appendages that permitted relatively rapid mobility in the ocean, to help gather food more efficiently and, more importantly, to escape other predators, probably played an important role in the sudden burst of animal life at this time. (1)

Dr Jean-Bernard Caron, a palaeontologist from the Royal Ontario Museum, is of the view that predators were probably the single biggest factor in pushing animals at this time to diversify into many varied forms. He said:

"It is during the Cambrian, starts with animals with legs and eyes, swimming. This didn't exist before. And this evolved very quickly at the beginning of the Cambrian.

We think that this evolution occurred relatively quickly because you find organisms that may have had some kind of different defence mechanism which has got to be response to higher predatory levels."

At the same time, as animals moved about more quickly and dared themselves to venture closer to the surface of the water, the eyes could have developed over time to become more accustomed to the light penetrating the water to help species to observe approaching predators and navigate coal reefs more easily.

Among the animals to emerge at this time include the first "animals with backbones" known as vertebrates. Thanks to the presence of bone material to support an internal skeletal structure, this material could not only help to protect vital internal organs, but also provide a means of supporting much larger muscles to help push the body through the water at reasonable speeds. The kind of speeds needed to out-swim a number of predators.

Not to be outdone in the evolutionary stakes, so too did those animals with an external skeletal system and no backbone, classified as early arthropods, of which crustaceans later emerged as a separate subgroup of species, such as lobsters. Again driven by the need for greater protection from whatever external forces were designed to shorten the lives of these animals. Either from UV radiation or predators, it seems there was a need to push animals to become more complex and diverse to help improve their chances of survival in the water.

530 MILLION YEARS AGO

Ancient fish with backbones (for greater mobility and as an effective internal protection for the nervous system and other vital organs, known as vertebrates) and hard-shelled organisms (for greater external protection from the ravages of physical attack from predators) definitely populated the seas (2). Could this be another survival mechanism? Just producer as many offspring as possible, so it does not matter if some are taken by predators. Could this have been the time when sex was invented to allow for these regular sexfests and successful breeding to take place?

Among the earliest evidence of fossilized vertebrates from this period is an animal called Myllokunmingia fengjiaoa, measuring 28 mm long and 6 mm high. Found in southern China by D.G. Shu from the Early Life Institute and Department of Geology at Northwest University in Xian Province, this tiny creature revealed bands of muscle down its body that must have been attached to something. The most logical explanation is a single hardened internal rod-like structure running along what is believed to be the animal's back. Of course, this may not have been the first animal to have a backbone (and perhaps was one of many species that evolved from an earlier type), but it is certainly the oldest known fossil to show this very structure of what is almost certainly a backbone of some rudimentary type. More details about this animal can be found in Shu's original scientific paper titled "Lower Cambrian Vertebrates from south China" published in Nature, Volume 402, 4 November 1999.

The success of the vertebrates in out-swimming and evading most predators allowed them to diversify into many different species after 500 million years. These included animals that, around 475 million years ago, developed jaws and teeth to grab and rip apart food. By 410 million years ago a subset of these animals had evolved two pairs of fins (e.g., Parayunnanolepis with its additional armour-plated skin covering most of its body and head, except for the tail). And, as the fins and body became larger and more muscular, the skeleton-like structure evolved to accept calcium phosphate as a means of reinforcing the strength and density of the long stringy skeletal material called collagen while still maintaining some level of flexibility (i.e., to allow sufficient bending when certain forces are applied to the material). Today, the descendants of these early vertebrates with jaws, four primitive limbs, and a tougher skeletal structure can be seen in all the fish species, the amphibians, reptiles and ultimately mammals, to which humans belong. We truly owe our existence and our basic body design, including our backbone, jaws and limbs, to these incredibly ancient vertebrates of the great Cambrian period.

As for those other organisms that rely on an external skeletal armour for internal support and protection, this was also the time of the emergence of the highly successful ocean floor scavengers and hard-shelled organisms known as trilobites. Hard shells are naturally formed by absorbing calcium carbonate and other minerals from the ocean to help give them their legendary strength. Because of their hardness, trilobites became one of the most successful creatures to roam the ocean floor. So successful, in fact, that scientists have found many fine examples of fossilised forms of this creature, along with the genetic variations of this species in different aquatic environments. As of 2012, scientists have identified over 50,000 different species of trilobites. Many more species are expected to be found locked away in the rocks of North Africa and other parts of the world. But it wasn't just the hard exoskeleton that gave these trilobites their evolutionary advantage. Most trilobite species also had the power of sight to observe their environment in incredible stereoscopic detail. While some trilobites made full use of the ocean floor in deeper waters without the need for sensitive eyes, many other species adapted to shallow seas and coastal regions where their eyes could develop and make full use of the sunlight penetrating the water to paint a realistic picture of the watery environment in which they lived.

Trilobites were not afraid of colder water. In fact, the largest trilobites ever to live on Earth, measuring up to a metre long, have been found in places along the African coast. Around this time, the African continent was positioned over the South Pole, which explains the colder conditions.

520 MILLION YEARS AGO

The fossilized remains of animals from this era are well-preserved due to the fact that the animals tended to live in shallow wasters and as storms swept across the seas, the sand and water were whipped up and buried a number of the animals alive. If buried deep enough in the sand, the bodies were more likely to be better preserved.