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 multi-
cellular 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.