The Proterozoic-Phanerozoic boundary was set at around 542 million years ago during the 1800s, to mark the appearance of the first animal fossils in the geological record. However the fossils of several hundred groups of animals in the earlier Proterozoic era have been identified since systematic study of those forms started in the 1950s. These earlier fossils are now known as the Ediacara biota. The Phanerozoic Eon is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic. The Paleozoic Era was itself around half the length of the Phanerozoic Eon, lasted from 542 to 252 million years ago, and is subdivided into six geologic periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.
The Cambrian Period lasted from 542 to 485 million years ago, and marked a profound change in life on Earth. The rapid appearance and diversification of multi-cellular lifeforms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla. The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the 1840s,(1)William Buckland (1841) Geology and Mineralogy Considered with Reference to Natural Theology and in 1859 Charles Darwin discussed it as one of the main objections that could be made against the theory of evolution by natural selection(2)Charles Darwin (1859) On the Origin of Species by Natural Selection, Pages 306–308 The long-term questions about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid change; and what it would imply about the origin of animal life.
Was the Cambrian Explosion a colonial event? Again, evolution of Proterozoic lifeforms into Cambrian lifeforms is a plausible explanation, and extraterrestrial intervention is not required. Unfortunately the Proterozoic fossils found do not appear to be related to Cambrian fossils, however we have an incomplete view of the Cambrian explosion and the Ediacara biota. If the Cambrian explosion was an alien intervention, then it would have happened around 33 million years after the potential intervention of the Avalonian Explosion. These aliens would need to be compatible with the biosphere already on the planet, yet would have followed a very different evolutionary path. Therefore this theoretical civilization could be another former colony of the mother-civilization. The disappearance of the Ediacara organisms at the same time as the appearance of the Cambrian organisms could indicate that the new visitors had a different diet than the potential Avalonian visitors.
While diverse life forms prospered in the oceans, the land was comparatively barren with nothing more complex than a microbial soil crust(3)Juergen Schieber et al. (2007) Atlas of Microbial Mat Features Preserved within the Clastic Rock Record, Pages 53–71 and a few molluscs appearing in the fossil record.(4)Adolf Seilacher and James W. Hagadorn (2010) Early Molluscan evolution: evidence from the trace fossil record. Palaios, Volume 25, Number 9, Pages 565-575 The continents are believed to have been dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the super-continent Pannotia. These seas and the oceans in general were relatively warm, and polar ice was absent for much of the period. This massive explosion of the number of species on the Earth actually took place in the beginning of the Cambrian Period, by 515 million years ago the number of species going extinct on the planet out numbered the new species appearing.
Around 500 million years ago, oxygen levels fell dramatically in the oceans, leading to hypoxia,(5)Michael Marshall (2011) “Oxygen crash led to Cambrian mass extinction” New Scientist while the atmospheric levels of poisonous hydrogen sulfide simultaneously increased, causing another extinction. The later half of Cambrian was remarkably barren, and shows evidence of several rapid extinction events. The stromatolites which had been replaced by reef building sponges known as Archaeocyatha, returned once more as the Archaeocyathids went extinct.
The Cambrian-Ordovician extinction events are not understood, however they seem to have been caused by a massive drop in the oxygen levels in the atmosphere and oceans. The generally increasing anoxia (lack of oxygen) of the Cambrian Era seems to have reached its zenith at approximately 500 million years ago, at a point scientists call the Steptoean Positive Carbon Isotope Excursion (SPICE). After the SPICE event, oxygen levels recovered and levels in the atmosphere may have risen as high as 30%, higher than the current 21%. This increase in oxygen is generally believed to have caused the increase in animal and plant species throughout the Ordovician Period.(6)Matthew R. Saltzman et al. (Feb. 21, 2011) Pulse of atmospheric oxygen during the late Cambrian, Proceedings of the National Academy of Science
The Cambrian-Ordovician extinction events appear to be similar to later prolonged extinction events, including the current 12,000 year long Holocene Extinction event. The current creation of anoxic dead-zones in the Earth’s continental shelves and sea floors are similar to the dead-zones found in the geological record. Our dead-zones are caused by nitrogen and phosphorus rich fertilizer run-off from the continents, and during the Cambrian Period there is no evidence for terrestrial farms or any significant terrestrial plants. The chief cause of the SPICE seems to be the sudden emergence of thousands of new plant and animal plankton species, many of which continue to be central to the world’s food-chain today. These species continue to serve as a crucial source of food to many large aquatic organisms, such as fish and whales. Plankton is also used as a source of food for humans, with krill being eaten in Japan since at least the 1800s. The Soviet Union was the world’s largest harvester of krill, peaking in 1983 with 490,000 tonnes harvested from Antarctic waters alone. After the collapse of the Soviet Union the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR) defined maximum catch quotas for a sustainable exploitation of Antarctic krill due to concerns that the oceanic food-chain could collapse.(7)Quirin Schiermeier (September 2, 2010) “Ecologists fear Antarctic krill crisis” Nature, Volume 467, Number 15, Page 15
It is possible that the sudden emergence of massive numbers of plankton species during the SPICE was a case of Directed Panspermia, and that the Earth’s oceans were being used as an aqua-farm. By 480 million years ago the first known giant filter-feeding animals were swimming through the Earth’s oceans, including the 2.1 meter (7 foot) long Aegirocassis benmoulae,(8)Peter Van Roy et al. (June 4, 2015) “Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps” Nature, Volume 522, Pages 77–80 which indicates the abundance of plankton in the Earth’s oceans at the time, and itself could have been a valuable food source for a neighboring civilization.
References [ + ]
|1.||⇑||William Buckland (1841) Geology and Mineralogy Considered with Reference to Natural Theology|
|2.||⇑||Charles Darwin (1859) On the Origin of Species by Natural Selection, Pages 306–308|
|3.||⇑||Juergen Schieber et al. (2007) Atlas of Microbial Mat Features Preserved within the Clastic Rock Record, Pages 53–71|
|4.||⇑||Adolf Seilacher and James W. Hagadorn (2010) Early Molluscan evolution: evidence from the trace fossil record. Palaios, Volume 25, Number 9, Pages 565-575|
|5.||⇑||Michael Marshall (2011) “Oxygen crash led to Cambrian mass extinction” New Scientist|
|6.||⇑||Matthew R. Saltzman et al. (Feb. 21, 2011) Pulse of atmospheric oxygen during the late Cambrian, Proceedings of the National Academy of Science|
|7.||⇑||Quirin Schiermeier (September 2, 2010) “Ecologists fear Antarctic krill crisis” Nature, Volume 467, Number 15, Page 15|
|8.||⇑||Peter Van Roy et al. (June 4, 2015) “Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps” Nature, Volume 522, Pages 77–80|