Chapter 16: The Origin of Life and the Precambrian EvolutionOverview |
In this chapter we turn, as Darwin did at the end of On the Origin of Species, to the big picture of life on Earth. Turn to Chapter 16 of your textbook to review work by scientists attempting to answer some of the most intriguing, profound, and difficult questions in biology.
16.1 What Was the First Living Thing? In the early 1980s, two teams of scientists independently discovered small enzymes that could break and reform the chemical bonds that link nucleic acids into chains. The enzymes did their job poorly; compared to the hundreds of other such enzymes already known, they were slow at their catalytic task and showed little versatility. Yet the discovery has been recognized as among the most significant biological breakthroughs in the last 20 years.
Refer to section 16.1 to learn about this and other important investigations into the origin of life.
16.2 Where Did the First Living Thing Come From? The RNA World has many attractive features, and it solves the problem of having to propose the advent of proteins before DNA existed to encode them. But an RNA World comes with troubles of its own. Some critics have claimed that it simply pushes the problem of the origin of self-replication one step back, much like the Panspermia Hypothesis pushes the problem of the origins of life off the earth to some other location.
Turn to section 16.2 of your textbook to explore the question of how RNA sequences came into being in an abiotic environment?
16.3 What Was the Last Common Ancestor of All Extant Organisms and What Is the Shape of the Tree of Life? Once self-replicating systems evolved on Earth, at least one of them adapted to the use of DNA to store heritable information and to the use of proteins to ex-press that information. This system eventually gave rise to all lineages of life on the planet today. We draw this conclusion because all life forms (except some viruses) use DNA and proteins. In fact, all modern organisms use them in the same way; the same 20 amino acids and the same basic structure of the genetic code have been found in all creatures studied to date.
Turn to section 16.3 of your textbook to learn about the principle of parsimony and how it is used to infer that all organisms share a common ancestor.
16.4 How Did the Last Common Ancestor’s Descendants Evolve into Today’s Organisms? The whole-life phylogeny suggests that the last common ancestor was a prokaryote. That is, a simple cell lacking a nucleus and organelles. Horizontal gene transfer notwithstanding, this is a reasonable hypothesis. At the cellular level, evolution from the cenancestor to organisms like ourselves has been a story of acquisition of complexity.
Refer to section 16.4 of your textbook to explore how today’s organisms evolved from a common ancestor.