Lecture 16: Life on Earth—Single-celled Organisms

Life on Earth—Single-celled Organisms...........................................

LECTURE

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.

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 À u.

Like all cells, prokaryotes have a fatty membrane through which chemicals

can À ow inward (for nutrition) and outward (for excretion). Within the cell

there are free-À oating 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 seaÀ oor or by consuming other prokaryotes.

The second transition is the evolution of the complex chemical reaction

known as “photosynthesis.” Photosynthesis is an extremely complex