The Origin and Evolution of Early Life

Chapter Summary

1. Understanding the origin of life requires interdisciplinary collaboration among biologists, chemists, geologists, and atmospheric scientists.
2. At the base of the tree of life is the last universal common ancestor (LUCA). LUCA was not a single organism, but a population of organisms. LUCA was not the first life-form, or the only life-form present at the base of the tree of life. But, by definition, it is the only one that left any descendant lineages that remain to this day.
3. When we use phylogenetic analysis, we cannot see back beyond LUCA, for LUCA is a common ancestor to any group of living species that we might choose to analyze. LUCA represents a phylogenetic event horizon: a point in the history of life beyond which phylogenetic analysis cannot possibly see.
4. Properties of living organisms include homeostasis, structural organization, metabolism, growth and reproduction, and the ability to respond to environmental conditions or stimuli. In addition, all life is subject to the process of evolution by natural selection.
5. The origin of life was more than just the origin of self-replicating entities; heritable variation for natural selection to operate on was also necessary.
6. In the 1920s, Oparin and Haldane proposed the prebiotic soup theory for the origin of life. Miller and Urey tried to simulate the conditions outlined by Oparin and Haldane. From their simple experimental protocol, Miller and Urey produced some of the building blocks of life: amino acids.
7. From about 4 billion to 3.5 billion years ago, life may have been based on RNA rather than on DNA. Ideas on the RNA world have been experimentally tested. Work in this area made a huge leap forward with the discovery of the first RNA enzymes: ribozymes.
8. Evolutionary biologists have built mathematical models and conducted experiments to simulate the conditions of the RNA world, in part to find a bridge from the RNA world to a world in which life is dominated by DNA and protein.
9. Molecular mutualisms may have been important among replicators in the RNA world and may have been critical in the evolution of early cells. The hypercycle model was constructed to address this possibility.
10. The origin and early evolution of bacteria were accelerated by what is known as horizontal gene transfer (HGT). HGT of genes or gene clusters may be especially important with respect to modular cell functions—those not extensively integrated with other functions in a cell. Depending on the extent of HGT, early cell life might resemble a hodgepodge of different cell forms readily exchanging genetic information.
11. Using genome analysis and experimental manipulations, scientists are attempting to understand early cellular evolution by calculating the minimal characteristics that a cell would need to operate as a living organism.