4: Forces of Evolution
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Learning Objectives
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It’s hard for us, with our typical human life spans of less than 100 years, to imagine all the way back, 3.8 billion years ago, to the origins of life. Scientists still study and debate how life came into being and whether it originated on Earth or in some other region of the universe (including some scientists who believe that studying evolution can reveal the complex processes that were set in motion by God or a higher power). What we do know is that a living single-celled organism was present on Earth during the early stages of our planet’s existence. This organism had the potential to reproduce by making copies of itself, just like bacteria, many amoebae, and our own living cells today. In fact, with modern technologies, we can now trace genetic lineages, or phylogenies, and determine the relationships between all of today’s living organisms—eukaryotes (animals, plants, fungi, etc.), archaea, and bacteria—on the branches of the phylogenetic tree of life (Figure 4.1).
Looking at the common sequences in modern genomes, we can even make educated guesses about the likely genetic sequence of the Last Universal Common Ancestor (LUCA) of all living things. Through a wondrous series of mechanisms and events over nearly four billion years, that ancient single-celled organism gave rise to the rich diversity of species that fill the lands, seas, and skies of our planet. This chapter explores the mechanisms by which that amazing transformation occurred and considers some of the crucial scientific experiments that shaped our current understanding of the evolutionary process.
This chapter is a revision from “ Chapter 4: Forces of Evolution ” by Andrea J. Alveshere. In Explorations: An Open Invitation to Biological Anthropology, first edition , edited by Beth Shook, Katie Nelson, Kelsie Aguilera, and Lara Braff, which is licensed under CC BY-NC 4.0 .