560-540 MILLION YEARS AGO
Another explosion of mostly invertebrate multi-celled animal lifeforms of enormous variety in the oceans, inland seas, and of the various lakes peppered over the desert-like expanses inland took place close to around 542 million years ago.
Why the sudden explosion in diversity of animal life? Scientists are not entirely sure, but one thing is certain: the development of specialised appendages that permitted reasonably rapid mobility in the ocean to help gather food more efficiently and, more importantly, escape other predators probably played an important role in the sudden burst of animal life at this time. (1)
It was probably also the time when the first "animals with backbones" known as vertebrates appeared on the scene. Thanks to the presence of bone material to support an internal skeleton structure, not only could this material help to protect internal organs, but also provide a means of supporting much large muscles to help push the body through the water at reasonable speeds.
Not lagging behind the evolutionary stakes, so too did those animals with an external skeletal system and no backbone classified as crustaceans evolve into more complex and diverse species. Something was making the animals evolve quicker.
Among the various means of protecting animals from predators included the use of an inorganic compound known as calcium carbonate in particle form to bind together with organic compounds of protein and biopolymers forming a tough outer material called shells. In this image, some of the animals are hidden inside these spiral-shaped shells. Other animals used different methods to protect themselves, such as thorny spikes around the body, or made full use of hide away places behind rocks, underneath the sands, or inside underwater caverns as well as the increasing plant life on the ocean floor. For the majority of animals, greater speed through the water became integral. Is this how the rudimentary formation of an internal backbone first came to be? Image © 2013 David Attenborough's Rise of Animals: Triumph of the Vertebrates (BBC and Atlantic Productions Ltd.).
There could be an explanation for this massive explosion in life. As animals moved closer to the surface of the oceans (a natural means of evading predators moving about along the ocean floor where most food can be found), the UV radiation may have assisted in creating many more mutations, of which the worse kinds were weeded out by natural selection and the better ones survived. It is believed this led to a bewildering array of animal species 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 observe the presence of other marauding creatures moving through the prehistoric waters (especially the predators), better means of gathering food from the ocean floor and quickly hiding again, and better appendages and body shapes for swimming through the oceans.
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."
Possibly the oldest fossil of an invertebrate found in South Australia has been dated to around 560 million years ago.
Earth in the Cambrian period around 548 million years ago. Image © 1997 Christopher R. Scotese. As of 2014, an updated map can be downloaded from the Colorado Plateau Geosystems, Inc. web site and created by Professor Ronald C. Blakey of Northern Arizona University (NAU).
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) proliferated in the seas. (2)
About 550 million years ago, marine invertebrates suddenly increased in numbers and types, including Trilobites and Brachiopods (Source: Reader 1986, p.53.), but already the earliest fossils of animals with primitive backbones known as the vertebrates are now apparent from this time.
Among the earliest evidence of fossilized vertebrates dated to 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 the Province of Xian, this tiny creature revealed bands of muscles down its body which had to be attached to something. The most logical explanation is a single hardened inner rod-like structure running along what is believed to be the animal's back. Of course, this may not be the first animal to have a backbone (and perhaps was just one species that evolved from an earlier type), but it is certainly the oldest known fossil containing 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 scholarly article titled "Lower Cambrian Vertebrates from south China" published in Nature, Volume 402, 4 November 1999.
Top picture of myllokunmingia fengjiaoa fossil discovered in the Burgess Shale-type deposits in south China by D.G. Shu, and below is a relatively accurate digital reconstruction of what it probably looked like (minus the skin colour) when it was alive. Today, the nearest living relative to this ancient creature are filter-feeding lampreys. Images © 2013 David Attenborough's Rise of Animals: Triumph of the Vertebrates (BBC and Atlantic Productions Ltd.).
The success of the vertebrates in outrunning and/or evading most predators allowed these tiny animals to diversify into many different species after 500 million years. Among them included animals that developed jaws and teeth to grab and rip apart food around 475 million years ago. By 410 million years ago a subset of these animals evolved two pair of fins (e.g., parayunnanolepis with its additional armour-plated skin covering most of its body and head except the tail) that would later develop into arms and legs for land-based animal such as humans. 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 skeleton 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 those early vertebrates with jaws, four primitive limbs, and tougher skeleton structure can be seen in all the fish species, the amphibians, reptiles and ultimately mammals of which humans are a part. We truly owe our existence and fundamental body design including our backbone, jaws and limbs to these incredibly ancient vertebrates of the great Cambrian period.
As for those other organisms relying on an external skeleton armour for internal support and protection, this was also the time for the emergence of the highly successful ocean floor scavenger and hard-shelled organism known as the 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 have been found fossilised in greater numbers allowing scientists to see relatively obvious genetic variations in this species in different watery environments. As of 2012, scientists have identified over 50,000 different trilobite species. Many more species are expected to be found locked away in the rocks of northern Africa and other places.
And it isn't just the hard-shelled exo-skeleton that gave these trilobite species their evolutionary advantage. Most trilobite species also had the power of sight to observe the 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 view of the watery environment they lived in.
Trilobites were not afraid of colder waters. In fact, the biggest trilobites measuring up to a metre long have been found in places along the African coastline. At around this time, the African continent was positioned over the South Pole, thus explaining the colder conditions.
Earth in the Late Cambrian era around 514 million years ago. Image © 1997 Christopher R. Scotese.