In science, the tree of life is used as a platform to describe the relationship of living things on planet earth from an evolutionary point of view. Not only does the universal phylogenetic tree cover all extant life, but it also covers the scope of the root and the earliest offshoots of stages in the process of evolution (Woese, 2000). This definition covers the time before cells in existence today were a reality. Several factors characterize evolution. Among these, homology and homoplasy are two important facets of evolution. In homology, two structures with similar functions are derived from a common origin. According to the tree of life, evolution of the cell as understood today is a product of the interaction between vertically derived and horizontally acquired variation (Abouheif, 1997). Modern cells are complex compared to their predecessors that were simpler and had a modular design. As a result, in the early days, there was widespread horizontal gene transfer that dominated the dynamics of evolution. The basis of the universal phylogenetic tree is a symbol of the initial stage in cellular evolution. At this stage, the evolving cell was adequately incorporated and could withstand the erosive effects of horizontal gene transfer in a manner that allowed true organismal lineages to occur (Gould, 1986).
The Cambrian Explosion
Around 530 million years ago, there was an apparent rapid influx of most major complex animals (Morris, 2000). This period as identified by fossil records is commonly known as the Cambrian explosion or radiation. It was also followed by a significantly large diversification in other organisms that encompassed phytoplankton, animals and calcimicrobes (Gould, 1986). In the pre-Cambrian period, there mainly existed simple organisms that were made up of single cells that were at times bunched in colonies (Gould, 1986). In the 70 or 80 million years that followed, the frequency of evolution sped up in relation to the extinction of earlier simple species and the diversity in living organisms that is seen today as Gould further observes.
A lot of scientific discussion has surrounded the Cambrian explosion. The apparent fast advent of fossils in the Primordial Strata was in the attention of scientists from as far back as the middle of the 19th century. For Charles Darwin, he considered it as one of the main points that would be used to disagree with his theory of evolution by natural selection (Woese, 2000). It has been a long running mystery on the abrupt appearance of the Cambria fauna. The mystery is based on three main points: if there was truly a rapid mass diversification of complex organisms in a short time in the early Cambrian, the cause of such a fast-paced evolution, and what that would signify for the origin and evolution of animals (Woese, 2000). It is difficult to determine the truth due to limited fossil records and chemical signatures found on Cambrian rocks.
It is possible to consider the Cambrian explosion as an expansion of the metazoans into empty niches in two waves (Morris, 2000). To begin with, the Ediacaran sea floor was the site of a joint evolutionary increase in the diversity of animals. There was a second enlargement in the early Cambrian that saw more animals diversify into the water column (Gould, 1986). There is no comparable outburst in rate of diversification among marine animals than was experienced in the Cambrian phase as Gould further observes. It had a profound impact on metazoan clades based on where Cambrian fossils have been found. Preceding radiations such as that experienced by fish in the Silurian and Devonian periods had less taxa involved and they mostly had body plans that were alike.
After the Permian-Triassic extinction, recovery began with as much animals as the Cambrian period (Abouheif, 1997). However, the period of recovery after the Permian-Triassic extinction produced much less new animals than the Cambrian explosion. The causing event of the Cambrian explosion set in motion a chain of events that opened up previously nonexistent ecological niches. Following their occupation, there was no more space for more similar widespread diversification to take place. This was due to high competition in the niches with existing occupants possessing advantage.
Variation in Rate of Species Evolution
A combination of various techniques including new developments in molecular genetics has advanced the study of fossil records significantly (Tavares et al., 2002). From studies, it has been established that some species did not undergo much evolution compared to other species. As Tavares et al., (2002) point out, a combination of factors have been attributed to this observation. Point mutation and infection by viruses have been claimed to contribute to the observation that some species failed to undergo as much evolution as their counterparts (Tavares et al., 2002). In the early stages, the DNA was easily attacked by viruses (Tavares et al., 2002). Natural selection also supports the proliferation of evolution in some species more than others (White, 2003). Natural selection has been active in the form of production of excessive offspring, steadiness of numbers, the continuous battle for existence in which some members of a species have to beat the odds to survive and only those that beat the odds get to breed (White, 2003).
Another factor that contributed to the observation in higher diversity in evolution in some species more than others is the variation of the offspring and the elimination of inferior lines through survival for the fittest (White, 2003). Individuals that did not have the ability to survive the existing conditions died off and could not breed. The individuals that could withstand the conditions bred leading to formation of new species that could flourish. Inheritance of a single variation was not reason enough to lead to the development of a new species (White, 2003). On the other hand, the introduction of several variations over multiple generations increased the likelihood of the eventuality of a new species.
The tree of life is an entrance to understanding the past and the future of life and evolution. The tree of life does exist and has the potential to address to major challenges that are faced by biologists. The challenges the tree of life can address are the history of the cells evolution and remaking the biological past of life as it is today. With the tree of life, it is possible to gain an understanding of the past and prediction of the future. Advances in science that have contributed towards an understanding of the tree of life include cell theory and Darwinian evolution. There is increasing evidence that horizontal gene transfer occurs in prokaryotes in both single cell and multicellular level. Subsequently, in as much as the tree of life explains the evolution of life, it does not provide a clear picture of the situations complexity but with advances in cell theory and molecular genetics, it can be used to get closer to the answers.
Abouheif, E. (1997). Developmental genetics and homology: a hierarchical approach. Trends in
Ecology & Evolution, 12(10), 405-408.
Gould, S. J. (1986). Evolution and the triumph of homology, or why history matters. American
Morris, S. C. (2000). The Cambrian explosion: Slow-fuse or megatonnage?.Proceedings of the
National Academy of Sciences, 97(9), 4426-4429.Tavare, S., Marshall, C. R., Will, O., Soligo, C., & Martin, R. D. (2002). Using the fossil record
to estimate the age of the last common ancestor of extant primates. Nature, 416(6882), 726-729.
White, T. (2003). Early hominids--diversity or distortion?. Science,299(5615), 1994.
Woese, C. R. (2000). Interpreting the universal phylogenetic tree.Proceedings of the National
Academy of Sciences, 97(15), 8392-8396.
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