Life on Earth—Single-celled Organisms

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.

We 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 u.

Like all cells, prokaryotes have a fatty membrane through which chemicals can ow inward (for nutrition) and outward (for excretion). Within the cell there are free- 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 sea 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

chemical reaction found, today, in all plants and plant-like organisms that contain chlorophyll. Organisms capable of photosynthesis can capture and store energy directly from sunlight, in a reaction whose main inputs are carbon dioxide and water and whose main outputs are sugary molecules such as glucoses, which can store energy, and free oxygen. The rst photosynthesizing prokaryotes appeared as early as 3.5 billion years ago. Photosynthesis marks a fundamental threshold in the history of life because it enabled living organisms to tap the colossal energy ows generated in the core of the Sun by hydrogen fusion.

Today, all plants practice photosynthesis, capturing energy from the Sun using green molecules of chlorophyll. As plants are consumed by other organisms, this captured energy diffuses throughout the biosphere via the “food chain.” Some of the oldest microfossils (3.5 billion years old) are photosynthesizing algae, like modern “cyanobacteria.” They created coral-like structures called stromatolites, some of which still exist today. Photosynthesis produces oxygen as a by-product. By 2.5 billion years ago, free oxygen started building up in the atmosphere. For many prokaryotes, oxygen was poisonous, which is why Lynn Margulis and Dorion Sagan described this change as the “oxygen holocaust” in their book Microcosmos. This revolutionary change in the atmosphere provides one marker for the beginning of the Proterozoic eon, from about 2.5 billion years ago.

The third crucial transition is the appearance of “eukaryotic” cells more than 1 billion years ago, during the Proterozoic eon. Lynn Margulis (1938–) showed that eukaryotes evolved through the merging of once independent species of prokaryotes. Evidence for this is the presence in all eukaryotes of internal “organelles,” some of which have their own DNA, which suggests they had once existed quite independently. Internal organelles include mitochondria, which can extract energy from oxygen, and chloroplasts, which can extract energy from sunlight through photosynthesis. The merging of these entities through “symbiosis” anticipates the later

Photosynthesis marks a fundamental threshold in the history of life because it enabled living organisms to tap the colossal energy ows generated in the core of the Sun by hydrogen fusion.

creation of multi-cellular organisms, though in the case of eukaryotes all the organisms coexisted within a single cell. Most eukaryotes are 10 to 1,000 times larger than prokaryotes; some can be seen just with the naked eye. The DNA of eukaryotes is protected within a special container, the nucleus, which limits the damage to genetic material and increases the accuracy of reproduction.

Many eukaryotic cells contain mitochondria, special “organelles” that can generate energy from oxygen—a more powerful source of energy than “fermentation,” the reaction used to generate energy in prokaryotes. So eukaryotes ourished in an oxygen-rich atmosphere. The appearance of eukaryotes marks a signi cant increase in the complexity of life. Lynn Margulis and Dorion Sagan write, “The difference between the new cells and the old prokaryotes in the fossil record looks as drastic as if the Wright Brothers’ Kitty Hawk ying machine had been followed a week later by the Concorde jet” (Margulis and Sagan, Microcosmos, p. 115).

The fourth crucial transition is the appearance of sexual reproduction about 1 billion years ago. Prokaryotes regularly exchange genetic material, but they normally reproduce simply by splitting into two identical individuals or “clones.” In most eukaryotes, two organisms exchange genetic material before reproduction, so that offspring contain a mix of genetic material from two parent individuals. As a result, offspring are no longer simply clones of their parents. Sexual reproduction introduces greater variation between individuals. As natural selection “selects” from such variations, the result of sexual reproduction is to speed up the rate of evolution. This is why evolution seems to have accelerated during the last 1 billion years.

This lecture has surveyed the rst 3.5 billion years of the history of life on Earth, during which all organisms were single-celled. How were multi-celled organisms created, and how did they evolve over the last 600 million years? That is the subject of the next lecture.

Essential Reading

Supplementary Reading

Questions to Consider

Brown, Big History, chap. 2. Christian, Maps of Time, chap. 5. Fortey, Life: An Unauthorised Biography.

Gould, The Book of Life. Margulis and Sagan, Microcosmos.

1. What were the most important changes in the history of life before the appearance of multi-celled organisms?

2. Why does photosynthesis count as such an important development in the history of life on Earth?