Life on Earth—Single-celled Organisms Lecture 16
Life for the best part of 3 billion years of the Earth’s history consisted of single-celled organisms. Not until about 600 million years ago would the ¿rst multi-celled organisms appear.
Lecture 16: Life on Earth—Single-celled Organisms
W
e don’t know how many species of living organisms there are today. There could be 10 million, or perhaps as many as 100 million. Only about 1 million have been described and catalogued. How, from simple beginnings, did this staggering variety of organisms evolve through natural selection? This and the next lecture describe how living organisms evolved to create the modern biosphere: the thin ¿lm of living organisms that covers the Earth’s surface. We will survey eight stages in the history of the biosphere, each of which created one of the elements that de¿ne our own species. This lecture describes the ¿rst four of these stages. It describes how life evolved and changed during the ¿rst 3.5 billion years of the Earth’s history, before the appearance of multi-celled organisms. The ¿rst organisms on Earth were single-celled “prokaryotes.” Prokaryotes are extremely simple cells. They are invisible to the naked eye. Indeed, countless billions live in or on our bodies. However, they are not the simplest of organisms. We have seen that viruses have evolved in the direction of greater simplicity, by shedding the capacity to generate energy on their own. They survive by hijacking the metabolic machinery of other organisms— something we experience, painfully, every time we come down with the Àu. Like all cells, prokaryotes have a fatty membrane through which chemicals can Àow inward (for nutrition) and outward (for excretion). Within the cell there are free-Àoating molecules of DNA. Though simple by some standards, even prokaryotes are immensely complex entities, full of constant frenetic chemical activity. The earliest prokaryotes probably got most of their food from chemicals near the seaÀoor or by consuming other prokaryotes. The second transition is the evolution of the complex chemical reaction known as “photosynthesis.” Photosynthesis is an extremely complex 72
