Sex and the Cambrian Explosion

Another supposed refutation of the Theory of Evolution that the Creationists like to present is the Cambrian Explosion. It is the first item at the top of the page on Fossil Evidence. The most straight forward explanation is that the Cambrian Explosion was the result of living organisms having sex.

I am not kidding! Some of the fossil impressions from the Ediacaran Period (635-540 mya), which preceded the Cambrian Period (540-490 mya), have been shown to have the morphology of embryos. The Ediacaran and Cambrian periods are the transition between the Proterozoic Eon and Phanerozoic Eon -- the current Eon of Geologic Time. The dominant forms of life in the Proterozoic Eon were unicellular Protists that reproduced by mitosis. Mitosis is similar to the process of binary fission that Bacteria and Archaea use to replicate themselves. However, as a method of replication, mitosis is less prone to random errors. The new types of errors that can happen during mitosis are translocation, inversion and chromosomal duplication. These types of mutations are non-random in the sense that they involve entire DNA sequences being recombined with each other in new ways rather than point mutations within a DNA sequence. During mitosis, new genes can arise out of the recombined functional subunits of other genes. Sexual reproduction is different again in that it results in offspring that are not replicas of the parent organisms. Sexually reproducing organisms alternate between a diploid and a haploid state. This back and forth transition is carried out by the processes of meiosis and fertilization. Sexuality introduces genetic recombination at the chromosomal level and provides a starting point for the development of a new organism.

Meiosis separates the paired chromosomes of the diploid form of the organism into two haploid sets. Each of the resulting haploid set is a random combination of the chromosomes derived from each parent. During meiosis the same mutations of translocation, inversion and chromosomal duplication can occur as in mitosis. However, chromosomal crossover is another form of mutation unique to meiosis. Parts of a chromosome derived from one parent can be exchanged with parts of the complimentary chromosome derived from the other parent. In this way, new chromosomes can arise that are recombinations of parts of earlier chromosomes. This is non-random mutation at a chromosomal level analogous to the genetic mutations that occur during mitosis.

Fertilization recombines the haploid sets of chromosomes resulting from meiosis into a new diploid form of the organism, not a replica of either of the parent organisms. It is important to understand that fertilization is the beginning of a new genetically unique organism. Archaea, Bacteria and other Protists that replicate by binary fission or mitosis are incapable of developing into complex multicellular organisms. Their multicellular forms are colonial aggregations of cells. Sexually reproducing organisms are able to develop into complex forms that have differentiated cells, specialized tissues, functional organs and integrated organ systems because every occurrence of fertilization is new evolutionary starting point upon which the process of natural selection can act. Each new organism will survive long enough to reproduce or it will not. Muticellular organisms are survival machines that serve to perpetuate the genetic material that they are an expression of, and populations of multicellular organisms constitute a gene pool that new combinations of genes are drawn from to create variations in the forms and functions of individuals at all levels of their being.

If you have had the patience to follow me this far, I congratulate you! The appeal of Creation Science is that it offers simplistic answers to complex questions. The transition between the Ediacaran Period and Cambrian Period is marked by the appearance of animals with hard body parts that were easily preserved as fossils. The seemingly sudden appearance of a diversity of phyla and divisions in the fossil record is an artifact of the rarity with which impressions of soft bodied muticellular animals, algae and fungi are preserved. It is tempting to stop at that simplistic answer without trying to illuminate our ignorance of how even soft bodied muticellular organisms evolved. Throughout the Archaean Eon (3800-2400 mya) and well into the Proterozoic, the dominant life-forms on Earth were stomatolites -- colonies of bacteria. During the Proterozoic Eon (2400-540 mya), after the oceans and atmosphere became oxygenated, unicellular Protists slowly evolved many divisions of larger and more complex types of cells. Then, during a relatively brief period of transition spanning the Ediacaran and Cambrian periods (635-490 mya), an amazing diversity of multicellular organisms appeared. The distinguishing characteristic of the multicellular organisms that have dominated the Phanerozoic Eon (540-0 mya) is sexual reproduction. Because sexuality increases the range of forms that individuals can exist in and introduces new forms of genetic mutation and recombination, the rate of evolutionary change accelerated. So, when animals first evolved the ability to form hard body parts at the beginning of the Cambrian Period, there had already been at least 95 million years of accelerated evolution, resulting in the diversity of forms that seemed to appear suddenly in the fossil record.

Something that has always fascinated me about Geologic Time is how relatively short the Phanerozoic Eon has been. And yet, during that time most of the diversity of life as we know it has evolved. The most eventful thing that happened during the evolution of asexual microbes was the production of oxygen. It was when they started having sex that the party really got going. ... ;-) ... lol ... Tell that to a Creationist!