Visions of the Cosmos
Life Among the Stars
The History of Earth - The Cambrian Explosion
In the previous page of the web-site we learnt how life began to appear on our planet about 3,800 million years ago. This page will deal with the very important changes which occurred during Vendian and more significantly the Cambrian periods and marked the change from unicellular organisms and very simple multicellular organisms to complex animals.
650 to 543 Million Years Ago
The latest period of the Proterozoic called the Vendian, or the Ediacaran, lasted from about 650 to 543 million years ago. Unlike later portions of the geologic time scale, the Vendian has no formal subdivisions nor distinct early boundary. This is in large part due to the fact that it has only recently become a subject of interest to paleontologists.
For many decades, paleontologists believed that life began in the Cambrian, or that if simpler life had existed in the Precambrian, that it left no fossil evidence for us to find.
In the mid-20th century macroscopic fossils of soft-bodied animals algae, and fossil bacteria have been found in these older rocks in a few localities around the world. With the discovery of these earliest fossils came a surge of interest in the Vendian and the Proterozoic Era that continues today
In 1946 peculiar Precambrian fossils were found by Reg Sprigg in the Ediacara Hills in South Australia. They were extensively studied in the 1950s by Martin Glaessner The name Ediacara was given to the group of fossils. It is the only geological period to have a name derived from the southern hemisphere and was derived from the Australian Aboriginee name meaning 'a place near water'. Since the first discoveries were made similar fossils have been found in Canada ( The Mackenzie Mountains in the Northwest Territories, The Yukon, British Columbia, and Newfoundland) and near the White Sea in Northern Russia..
The Vendian extinction occurred near
the close of the Vendian period, and largely affected
stromatolites has been correlated with a large glaciation
event that occurred about 600 million years ago. This event was of such
severity that it is thought that almost all micro-organisms were
completely wiped out.
The Vendian extinction is currently under debate as to whether an extinction event occurred or not. Many paleontologists believe that the Vendian fauna were the progenitors of the Cambrian fauna. However, others believe that the Vendian fauna have no living representatives. Under this latter hypothesis, the Vendian fauna is believed to have an undergone an extinction, after which the Cambrian fauna evolved. Until more information can be collected, details on the Vendian extinction event will remain open to debate.
The late Precambrian was a time of many changes - 800 million years ago the super-continent of Rodina dominated the Earth, by the start of the Cambrian this was breaking up into smaller land masses, increasing the area of continental shelf environment and producing shallow seas. Most readers will already be aware of plate tectonics and seafloor spreading and will know that the continental landmasses of the Earth's crust move slowly across the semi-liquid magma of the asthenosphere. This is shown in the Palaeogeographical map of early Cambrian times reproduced by courtesy of the Bristol University Dept of Earth Sciences.
.Photosynthetic organisms had reduced the amount of carbon dioxide in the atmosphere, causing the planet to cool down. According to the evidence for the 'Snowball Earth', worldwide glaciations of Late Riphean and Early Vendian periods occurred between 800-600 million years ago, ending with the harshest ice-age in Earth's history. Oxygen levels were also increasing due to the high biomass of photosynthetic organisms. After the glacial period, temperatures rose until by the time of the Cambrian itself the climate was warmer than it is today. In terms of geological time there was a sudden 'explosion' of the metazoa (animals) at the base of the Cambrian
These environmental changes over the Cambrian/Precambrian boundary potentially are the driving forces behind the Cambrian explosion event, many theories have been put forward trying to link the two together
|Rocks of the Vendian Period occur in the Ediacara Hills of Australia and in the White Sea Region of Russia. Soft bodied primitive animals left fossil traces. Modern examples of sponges No one is sure to what group of animals the ediacara belonged if there are any modern equivalents|
noctiluca, Connemara, W. Ireland
September 1999, Photograph by Jim Greenfield
|Dickinsonia is known from Vendian rocks of south Australia and north Russia. It is often considered to be an annelid worm because of its apparent similarity to one genus of extant polychaety, Spinther. However, in the opinion of some, it may in fact be a cnidarian polyp, like a soft-bodied version of the "banana coral," Fungia||Named Tribrachidium heraldicum, its affinities are still mysterious, although distant relationships have been proposed with either the Cnidaria(corals and anemones) or Echinodermata (urchins and seastars||The University of California Museum of Palaeontology
have a good collection of ediacara. 2 examples are shown of Dickinsonia
Credit Website www.berkeley.edu
The Cambrian Explosion
The rapid evolution of highly organised multicellular animal and plant life covers a relatively small period of the history of our planet.
Until recently there was almost no evidence for life before the Cambrian Period. About 543 million years ago an enormous variety of marine life suddenly appeared very rapidly in the stratigraphical records. In geological time this appearance of life forms was very rapid and is referred to as 'THE CAMBRIAN EXPLOSION'. The record of life forms from then on was very clear right up to the present day. Rocks below the Cambrian in the stratigraphical records were simply referred to as Pre-Cambrian. In recent years much more has been discovered about Pre-Cambrian times. It is worth noting that the whole period of the Pre-Cambrian encompasses 88-90% of the History of the Earth and that highly organised life covers only about 10-12%.
The Cambrian Period
543 to 490 Million Years Ago
Illustrations from the book by Simon Conway Morris 1999 'The Crucible of Creation' of life in the Cambrian Seas
Reconstruction of Early Cambrian continental
In the Cambrian Explosion life suddenly and rapidly flourished some 540 million years ago may have it’s explanation in the reaction of primitive life to some big event. One of the most fruitful discoveries of deposits of Mid-Cambrian fossils was found in the Burgess Shale in Canada and has been extensively dealt with in a book entitled 'The Crucible of Creation' by Professor Conway-Morris. The book was published by the Oxford University Press in 1998 - ISBN Number 0 19 850256 7. Some of the illustrations in this web-site are taken from the book.
The animals in the picture on the right are dominated by various forms of worms. A trilobite can also be seen in the centre strolling across the sea floor and swimming through the water in the distance is a Pikaia which was a primitive chordate. The chordates are the creatures from which the vertebrates (the animals with backbones) evolved.
There are a number of animals which are connected to the floor of the sea bed. These animals are called sessile. There are a surprisingly large groups of animals which like plants are not free to move around their surroundings. The sessile animals in this scene include blue Vauxia sponges, and green Mackenzia cnidarian.
Credit for illustration of life in Cambrian times to University of Michigan Exhibit Museum, Carl Wozniak. (Earth History Resources)
Some scientists say the life explosion might just have easily occurred two billion years earlier - or not at all. Certainly it was a period of history that witnessed the assembly and break-up of two super continents and at least two major glaciation events. Atmospheric oxygen levels were also on the rise.
Computer models showed there were two zones of stability for the Earth - with or without higher life forms - and that about 542 million years ago the planet flipped from one to the other. What caused the flip is not clear. It might have been a continental break-up, a vast thaw following the world–wide glaciation or even an asteroid impact.
On Monday 13 October 2003 Dr David Whitehouse the BBC Online Science Editor discussed the Cambrian Explosion. He quoted from Dr Werner von Blow of the Potsdam Institute for Climate Impact Research.
Dr von Blow says that after the Cambrian Explosion there has been a stabilisation of temperature up to the present, and that the biosphere is not playing a passive role but a very active one. In this he supports the Gaia Hypothesis of James Lovelock and Lynn Margulis.
He also adds that there is an intriguing implication from his research which suggests that had the conditions been only slightly different, the Cambrian Explosion could have occurred two billion years earlier or alternatively not at all. In which case the Earth could still be inhabited by nothing more complex than bacteria.
An early explosion would have meant that by now the Earth could have developed far more advanced intelligent creatures than humans!
Dr von Bloh says that it will be of great interest if find other Earth-like worlds circling other stars to see if they have had their own Cambrian explosions yet.
The timing of such events has implications for the search for intelligent life in space, he says.
At the end of the Cambrian there was a mass extinction which was followed by the unfolding of the new life forms of the Ordovician.
Follow on to History of the Earth -The last 600 million years
The picture on the left emphasises that a large number of sessile animals were living in the Burgess Shale community Attached to the sea floor are various types of sponge
The large blue sponges belong to the Vauxia group. In the lower left there are Pirania with prominent projecting spikes. The Choia in the middle resemble small pin cushions. Further in the background are yellow Dinomischus, green Mackenzia and purple Chancelloria. The prominent group of three frond-like organisms on the mid-upper left are examples of the sea-pen, Thaumaptilon.
Moving across the sea floor are two trilobites
Follow on to History of the Earth -The last 600 million years