Lecture 17: Life on Earth—Multi-celled Organisms

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

LECTURE

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 single-

celled eukaryotic cells, organisms.

T

his 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