Those evolutionary biologists who are aware of the data establishing the challenges listed above acknowledge that the challenges defy all macroevolutionary models. Evolutionary biologists who do recognize the failure of evolutionary models to explain the Cambrian explosion often counter those of us who are evangelical Christian research scientists by asking us why God would choose to create Cambrian animals at the time and place that he did.
The answer that I document and describe at some length in Improbable Planet is that unless God created the Cambrian animals in the greatest possible diversity at the greatest possible abundance levels at the earliest time permitted by the laws of physics and the history of the universe, human beings and human civilization would be impossible.
Here is an an example from our inbox and my answer: Q: Have evolutionists come up with any valid explanation for the sudden appearance of the Cambrian explosion? Ten of the many challenges the Cambrian explosion poses to evolutionary explanations for life are as follows: While evolutionary scenarios, as opposed to worked-out theories, exist for hypothesizing how new genera, new orders, and new families of animal life might appear, there is no rational evolutionary scenario for explaining how a new animal phylum might appear.
Of the animal skeletal designs theoretically permitted by the laws of physics, appear in the Cambrian explosion fossils. The Cambrian explosion marks the first appearance of animals with skeletons, bilateral symmetry, appendages, brains, eyes, and digestive tracts that include mouths and anuses.
Virtually every eye design that has ever existed appears simultaneously in the Cambrian explosion. The rise of carnivory would have set off an evolutionary arms race that led to the burst of complex body types and behaviours that fill the oceans today. In the modern world, it's easy to forget that complex animals are relative newcomers to Earth.
Since life first emerged more than 3 billion years ago, single-celled organisms have dominated the planet for most of its history. Thriving in environments that lacked oxygen, they relied on compounds such as carbon dioxide, sulfur-containing molecules or iron minerals that act as oxidizing agents to break down food. Much of Earth's microbial biosphere still survives on these anaerobic pathways. Animals, however, depend on oxygen — a much richer way to make a living.
The process of metabolizing food in the presence of oxygen releases much more energy than most anaerobic pathways. Animals rely on this potent, controlled combustion to drive such energy-hungry innovations as muscles, nervous systems and the tools of defence and carnivory — mineralized shells, exoskeletons and teeth. Given the importance of oxygen for animals, researchers suspected that a sudden increase in the gas to near-modern levels in the ocean could have spurred the Cambrian explosion.
To test that idea, they have studied ancient ocean sediments laid down during the Ediacaran and Cambrian periods, which together ran from about million to million years ago.
In Namibia, China and other spots around the world, researchers have collected rocks that were once ancient seabeds, and analysed the amounts of iron, molybdenum and other metals in them. The metals' solubility depends strongly on the amount of oxygen present, so the amount and type of those metals in ancient sedimentary rocks reflect how much oxygen was in the water long ago, when the sediments formed. These proxies seemed to indicate that oxygen concentrations in the oceans rose in several steps, approaching today's sea-surface concentrations at the start of the Cambrian, around million years ago — just before more-modern animals suddenly appeared and diversified.
This supported the idea of oxygen as a key trigger for the evolutionary explosion. But last year, a major study 1 of ancient sea-floor sediments challenged that view. Erik Sperling, a palaeontologist at Stanford University in California, compiled a database of 4, iron measurements taken from rocks around the world, spanning the Ediacaran and Cambrian periods.
He and his colleagues did not find a statistically significant increase in the proportion of oxic to anoxic water at the boundary between the Ediacaran and the Cambrian. The latest results come at a time when scientists are already reconsidering what was happening to ocean oxygen levels during this crucial period. Donald Canfield, a geobiologist at the University of Southern Denmark in Odense, doubts that oxygen was a limiting factor for early animals.
In a study published last month 2 , he and his colleagues suggest that oxygen levels were already high enough to support simple animals, such as sponges, hundreds of millions of years before they actually appeared.
Cambrian animals would have needed more oxygen than early sponges, concedes Canfield. Sperling has looked for insights into Ediacaran oceans by studying oxygen-depleted regions in modern seas around the globe. He suggests that biologists have conventionally taken the wrong approach to thinking about how oxygen shaped animal evolution. By pooling and analysing previously published data with some of his own, he found that tiny worms survive in areas of the sea floor where oxygen levels are incredibly low — less than 0.
Food webs in these oxygen-poor environments are simple, and the animals feed directly on microbes. In places where sea-floor oxygen levels are a bit higher — about 0. The implications of this finding for evolution are profound, Sperling says.
The modest oxygen rise that he thinks may have occurred just before the Cambrian would have been enough to trigger a big change. The gradual emergence of predators, driven by a small rise in oxygen, would have meant trouble for Ediacaran animals that lacked obvious defences.
Studies of those ancient Namibian reefs suggest that animals were indeed starting to fall prey to predators by the end of the Ediacaran. When palaeobiologist Rachel Wood from the University of Edinburgh, UK, examined the rock formations, she found spots where a primitive animal called C loudina had taken over parts of the microbial reef.
Rather than spreading out over the ocean floor, these cone-shaped creatures lived in crowded colonies, which hid their vulnerable body parts from predators — an ecological dynamic that occurs in modern reefs 5.
C loudina were among the earliest animals known to have grown hard, mineralized exoskeletons. But they were not alone. Two other types of animal in those reefs also had mineralized parts, which suggests that multiple, unrelated groups evolved skeletal shells around the same time. Some C loudina fossils from that period even have holes in their sides, which scientists interpret as the marks of attackers that bore into the creatures' shells 6.
Palaeontologists have found other hints that animals had begun to eat each other by the late Ediacaran. In Namibia, Australia and Newfoundland in Canada, some sea-floor sediments have preserved an unusual type of tunnel made by an unknown, wormlike creature 7. The chordates are characterized by a nerve cord, gill pouches and a support rod called the notochord.
In the Cambrian fauna, we first see fossils of soft-bodied creatures with these characteristics. However, the living groups of vertebrates appeared much later. It is also important to realize that many of the Cambrian organisms, although likely near the base of major branches of the tree of life, did not possess all of the defining characteristics of modern animal body plans. These defining characteristics appeared progressively over a much longer period of time. Not all scientists accept the idea that the Cambrian Explosion represents an unusually rapid evolutionary transition.
The fossil record is notoriously incomplete, particularly for small and soft-bodied forms. Some researchers argue that the apparent rapid diversification of body plans is an artifact of an increase in the rate of fossilization, due in part to the evolution of skeletons, which fossilize more effectively. In many cases these, often very tiny, mineralized structures are all that are found as fossils.
There were major changes in marine environments and chemistry from the late Precambrian into the Cambrian, and these also may have impacted the rise of mineralized skeletons among previously soft-bodied organisms.
Most scientists are persuaded that something significant happened at the dawn of the Cambrian era and view the Cambrian Explosion as an area of exciting and productive research. For example, scientists are now gaining a better understanding of what existed before the Cambrian Explosion as a result of new fossil discoveries.
Recent discoveries are filling in the fossil record for the Precambrian fauna with soft-bodied organisms like those in the Ediacaran Assemblages found around the world. Some of the new fossil discoveries, in fact, appear to be more primitive precursors of the later Cambrian body plans.
The discovery of such precursors shows that the Cambrian organisms did not appear from thin air. Genomic studies provide further insights into the origins of the Cambrian Explosion.
Although the genetic divergence of organisms would have preceded the recognition of new body plans in the fossil record, accumulating genomic data is broadly consistent with the fossil record. The sudden change of the Cambrian Era was, in relative terms, not too sudden for the process of evolution.
The changes during the Cambrian Era did not occur over decades, centuries, or even thousands of years; they occurred over millions of years—plenty of time for evolutionary change.
However, for millions of years beforehand, body plans of animals had remained relatively constant. Not until this time period did a significant change occur. The remaining questions are: What triggered the Cambrian Explosion? And why did so much change occur at this time? Several different theories address the origin of the Cambrian Explosion, proposing that dramatic environmental changes must have opened up new niches for natural selection to operate upon.
Wittbrodt J. Ciliary photoreceptors with a vertebrate-type opsin in an invertebrate brain. Science : — Google Scholar. Arthur W. Ayala F. Rzhetsky A. Origin of the metazoan phyla: Molecular clocks confirm paleontological estimates. Proceedings of the National Academy of Sciences 95 : — Bengtson S.
Rasmussen B. Krapez B. The Paleoproterozoic megascopic Stirling biota. Paleobiology 33 : — Blair J. Hedges S. Molecular clocks do not support the Cambrian explosion. Molecular Biology and Evolution 22 : — Bottjer D. Davidson E. Peterson K. Cameron R. Paleogenomics of echinoderms. Bowring S. Grotzinger J. Isachsen C. Knoll A. Pelechaty S. Kolosov P. Calibrating rates of Early Cambrian evolution. Briggs D. Fortey R. The early radiation and relationships of the major arthropod groups.
Bromham L. Combining molecular and palaeontological data to defuse the Cambrian explosion. Hendy M. Can fast early rates reconcile molecular dates with the Cambrian explosion. Proceedings of the Royal Society B : — Butterfield N.
Secular distribution of Burgess-Shale type preservation. Lethaia 28 : 1 — Plankton ecology and the Proterozoic-Phanerozoic transition. Paleobiology 23 : — Canfield D. Poulton S. Narbonne G. Late-Neoproterozoic deep-ocean oxygenation and the rise of animal life.
Science : 92 — Cavalier-Smith T. Cell evolution and Earth history: Stasis and revolution. Philosophical Transactions of the Royal Society B : — Cloud P.
Reflections on the beginnings of metazoan evolution. Precambrian Research 31 : — Jr Some problems and patterns of evolution exemplified by fossil invertebrates. Evolution 2 : — Pre-metazoan evolution and the origins of the Metazoa. Pages 1 — Colosimo P. Hosemann K. Balabhadra S. Villarreal G. Dickson M. Grimwood J. Schmutz J. Myers R. Schluter D. Kingsley D. Widespread parallel evolution in sticklebacks by repeated fixation of Ectodysplasin alleles. Conway Morris S. Palaeontology 20 : — Caron J-B.
Halwaxiids and the early evolution of the lophotrochozoans. Darwin C. Origin of bilaterian body plans: Evolution of developmental regulatory mechanisms. Dickerson R. The structure of cytochrome c and the rates of molecular evolution. Journal of Molecular Evolution 1 : 26 — Dzik J.
Paleobiology 31 : — Erwin D. Valentine J. Jablonski D. The origin of animal body plans. American Journal of Science 85 : — Fedonkin M. Waggoner B. The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism. Nature : — Fike D. Pratt L. Summons R. Oxidation of the Ediacaran ocean. Wills M. Biological Journal of the Linnaean Society 57 : 13 — Glaessner M. Wade M. The late Precambrian fossils from Ediacara, South Australia. Palaeontology 9 : — Gould S. Wonderful Life NewYork Norton.
Gradstein F. Ogg J. Smith A. Grenier J. Garber T. Warren R. Whitington P. Carroll S. Current Biology 7 : — Early metazoan divergence is about million years ago. Journal of Molecular Evolution 47 : — Harvey T. Sophisticated particle-feeding in a large Early Cambrian crustacean. Venturi M.
Shoe J. A molecular timescale of eukaryotic evolution and the rise of complex multicellular life. BMC Evolutionary Biology 4 : 2. Hoffman P. Kaufman A. Halverson G. Schrag D. A Neoproterozoic snowball Earth. Hou X-G. Bergstrom J. Journal of Paleontology 78 : — Kirschvink J. Raub T. A methane fuse for the Cambrian explosion: Carbon cycles and true polar wander.
Comptes Rendus Geoscience : 65 — Levinton J. Dubb L. Wray G. Simulations of evolutionary radiations and their application to understanding the probability of a Cambrian explosion. Journal of Paleontology 78 : 31 — Lundin L.
0コメント