Life on Earth—Multi-celled Organisms

Life on Earth—Multi-celled Organisms

Lecture 17

There’s a sense in which we have to remind ourselves that we, too, from a certain point of view, are merely vast crowds of billions of singlecelled eukaryotic cells, organisms.

This lecture traces the evolution of multi-celled organisms during the last 600 million years. It describes four more transitions on the evolutionary pathway leading to our own species, Homo sapiens. The rst transition we discuss in this lecture is the appearance of multi-cellular organisms almost 600 million years ago. As late as the 1950s, most biologists thought that life itself rst appeared on Earth only in the Cambrian era, about 570 million years ago, because that was when the rst naked-eye fossils appeared. We now know that single-celled organisms had already existed for almost 3 billion years. What the Cambrian era really marks is the appearance of the rst multi-cellular organisms.

The evolution of multi-cellular organisms was a complex process. For such organisms to work, billions of cells had to cooperate and communicate with great precision. It was also necessary for them to be able to communicate with each other in some way, and for each cell to know its place and role in the functioning of the organisms as a whole. These are staggering organizational challenges. However, as we have seen, such challenges were not entirely unprecedented, for evolution can involve cooperation as well as competition. In fact, simpler forms of cooperation that do not count as multi-cellularity had already evolved. Even eukaryotes formed through a symbiosis between distinct types of prokaryotes.

Early forms of collaboration took several forms. Stromatolites, like coral reefs, formed from huge colonies of individual prokaryotes in which the colony provided some protection to each individual cell. Some sponges that look like single organisms will reassemble if passed through a sieve, so we must assume that each cell retains its independence. Particularly fascinating are slime molds, colonies of amoeba that can come together to form a single entity when times are tough and then break apart again when

conditions improve. None of these species count as multi-cellular organisms. Nevertheless, the growing interdependence and specialization of their cells point toward multi-cellularity.

Genuine multi-cellularity requires that all participating cells have identical genetic material in order to avoid competition between cells. In human beings, for example, a fertilized cell will create billions of clones, each with the same genes. Yet each cell can develop in different ways, depending on the chemical environment in which it nds itself, for different chemicals can activate different parts of a cell’s genetic code. In this way, genetically identical cells can develop into any of the 210 distinct types of cells in human bodies, from bone cells to liver cells to brain neurons. Specialization allows multicellular organisms to handle a wider range of functions than single-celled organisms.

Multi-cellularity allowed the construction of gigantic organisms. To a prokaryote, you or I might look like a vast, mobile version of the Empire State Building. Indeed, our bodies contain as many cells as there are stars in the Milky Way. Once the rst multi-celled organisms appeared, they evolved rapidly in an “adaptive radiation.” Adaptive radiations occur frequently in evolutionary history, when a new type of organism appears and rapidly evolves into a wide range of different species.

The art of classifying these different species (“taxonomy”) was pioneered by Carl Linnaeus in the 18th century. Taxonomy groups living organisms into many nested categories. Today, the largest generally recognized category divides all living organisms into two “superkingdoms,” prokaryotes and eukaryotes. Below that come the “kingdoms,” which include animals, plants, and fungi. Then come “phyla” (such as the chordates), “classes” (such as mammals), “orders” (such as primates), and nally “species” (such as human beings). Now we will discuss the evolution of those categories of organisms that included our own ancestors.

About 67 million years ago, right at the end of the Cretaceous period, an asteroid impact (the “Cretaceous event”) destroyed most large species, including dinosaurs.

The second major transition on the road to modern humans is the appearance of the rst vertebrates. We belong to the phylum of chordates, or vertebrates—organisms with backbones. The rst vertebrates evolved about 500 million years ago, in the Ordovician period, from worm-like ancestors. They were probably unimpressive sh-like creatures with no heart or brain, a bit like modern “lancelets.” All vertebrates have a front and back end, and a complex system of internal communications through nerve cells running along the spine. The vertebrates include sh, amphibia, reptiles, birds, and mammals.

The third major transition leading to humans is the movement of some multicellular organisms from the sea to the land. Plants and insects probably reached the land rst about 500 million years ago, during the Ordovician period. The rst vertebrates to leave the sea did so about 400 million years ago, during the Devonian period. Leaving the sea posed huge challenges. You needed a strong skeleton as water no longer supported your weight. You needed a tough skin to avoid drying out. You needed special apparatus enabling you to breathe oxygen directly rather than through the water. And you needed some way of reproducing in a watery environment so your offspring would not dry out. The rst vertebrates to live for prolonged periods on land were probably a bit like modern lung sh, which can survive for some periods on land if the ponds they live in dry out. Two important “classes” of vertebrates were:

The amphibia, which lived on the land permanently but returned to the water to lay eggs, and which evolved during the Devonian period; and

The reptiles, which evolved about 350 million years ago, during the

Carboniferous period, and laid their young in eggs protected within tough skins.

The fourth transition on the path toward modern humans is the appearance of the class of mammals about 250 million years ago, during the Triassic period. As the supercontinent of Pangaea was forming, the majority of living species vanished during the Permian period, from about 290 to 250 million

years ago. This mass extinction may have been caused by an asteroid impact, though at present this is by no means certain. Another possibility is that it was caused by the coming together of many once-separate regions to form a supercontinent within which many different species had to compete for fewer niches. The removal of so many earlier species created space for a rapid “adaptive radiation” of new species. Two important new groups of vertebrates were the dinosaurs and mammals, both of which appeared in the Triassic period, between 250 and 210 million years ago. The class of mammals contains furry, warm-blooded organisms that nurture their young within their mother’s body and feed them with milk. (Even humans have fur, though not much!) The earliest mammals were probably small, shrew-like animals that foraged for insects at night.

About 67 million years ago, right at the end of the Cretaceous period, an asteroid impact (the “Cretaceous event”) destroyed most large species, including dinosaurs. That there was such an impact was demonstrated only in the 1980s by geologist Walter Alvarez. Mammal species diversi ed rapidly in a new adaptive radiation, lling niches vacated by the dinosaurs. As part of this mammalian radiation there appeared a new order of mammals, the primates: tree-dwelling mammals with stereoscopic vision, hands designed to grasp, and larger brains. We’ll cozy up to the primates in the next lecture because we, too, are primates.

We have seen four crucial stages in the evolution of multi-celled organisms like ourselves. Each of the eight stages of evolution described in this and the last lecture contributed something to the makeup of our own species, Homo sapiens. We will see how in the next lecture.

Essential Reading

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

Supplementary Reading

Questions to Consider

Alvarez, T. Rex and the Crater of Doom. Gould, The Book of Life. Lovelock, Gaia.

1. Why did multi-celled organisms evolve so late in the history of life on Earth?

2. How great a role was played by chance in the evolution of the life forms that exist today?