Multicellular organisms can indeed be multicellular—a human has about 60 trillion eukaryotic cells and many more prokaryotic ones. But it’s not just cells that make up a human or a rose plant—it is groups of cells specialized into tissues and then groups of tissues in organs (e.g., the nerve cells in the brain or the petals in a flower) that have specific roles. As you will learn in Chapter 19, the embryonic development of tissues and organs takes place in several steps:

Even though single-celled organisms continue to be highly successful on Earth, over time complex multicellular organisms evolved, along with their division of biological labor among specialized cells. The transition from single-celled to multicellular life took a long time. Indeed, while there is evidence that single-celled organisms arose about 500 million to a billion years after the formation of Earth (see Chapter 4), the first evidence of true multicellular organisms dates from more than a billion years later. Multicellularity probably arose several times.

Studying the evolutionary origin of multicellularity is a challenge because it happened so long ago. The closest unicellular relatives of most modern animals and plants probably existed hundreds of millions of years ago. The transition from single-celled to multicellular organisms may have occurred in several steps:

Does this list look familiar? (See above for the origin of organs in the embryo.)

A key event would have been the evolution of intercellular communication, which is necessary to coordinate the activities of different cells within a multicellular organism.

We can visualize how the evolution of multicellularity might have occurred by looking at the “Volvocine line” of aquatic green algae (Chlorophyta). These plants range from single cells to complex multicellular organisms with differentiated cell clusters (Figure 7.17). Included in this range are a single-celled organism (Chlamydomonas); an organism that occurs in small cell clusters (Gonium); species with larger cell clusters (Pandorina and Eudorina); a colony of somatic and reproductive cells (Pleodorina); and a larger, 1,000-celled alga with somatic and reproductive cells organized into separate tissues (Volvox).

Question

Direct communication between cells allows them to rapidly share signals, which travel from one cell to another in a group. This can result in common activities for a group of cells, which is important for tissues.

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Chlamydomonas is the single-celled member of this group. It has two cellular phases: a swimming phase, when the cells have flagella and move about, and a non-swimming phase, when the flagella are reabsorbed (disaggregated) and the cell undergoes cell division (reproduction). Compare this with Volvox: most of the cells of this multicellular, spherical organism are on the surface; the beating of their flagella gives the organism a rolling motion as it swims toward light, where it can perform photosynthesis. But some Volvox cells are larger and located inside the sphere. These cells are specialized for reproduction: they lose their flagella and then divide to form offspring.

The separation of somatic and reproductive functions in Volvox is possible because of a key intercellular signaling mechanism that coordinates the activities of the separate tissues within the organism. Volvox has a gene whose protein product is produced by the outer, motile cells and travels to the reproductive cells, causing them to lose their flagella and divide. This gene is not active in species such as Gonium and Pandorina, which show cell aggregation but no cell specialization.