Unicellularity Vs. Multicellularity: Why We Bother With More Than One Cell

If you know anything about deep history, you’ve doubtless heard something about the Cambrian Explosion, 542 million years ago (MYA), when all those trilobites showed up with their animal body plans. In many accounts, this era is spoken of as the dawn of multicellularity.

In fact, the story is much more interesting. It’s turning out that multicellularity is an idea that has been “invented” multiple times during the evolution of life.

In the era preceding the Cambrian known as the Ediacaran (635-542 MYA), bizarre multicellular creatures roamed the planet with body plans that have no apparent relationship to Cambrian forms. Even stranger multicellular fossil forms have been found in India that date to 1700 MYA. And last week we learned of a wonderful fossil find in Gabon, where thumb-sized creatures resembling irregularly shaped “wrinkly cookies” have been dated to 2100 MYA.

The individual cells in these various extinct multicellular experiments were doubtless all organized in the same way. Evidence abounds that all life on earth shares common ancestry with single-celled organisms dating back to perhaps 3500 MYA, organisms that encoded their instructions in DNA, synthesized proteins using ribosomes, and carried out deeply conserved metabolic pathways.

Most planetary life since the original common ancestor has continued to be single-celled. To bring this point home:  each human body has ~10 trillion human cells and ~100 trillion bacterial cells living in the gut and other tissues. Another perspective: there are as many bacteria in a spadeful of garden soil as there are humans on the planet.

Not only do we now know of several multicellular inventions prior to the Cambrian, with many others doubtless awaiting discovery by fossil hunters. Multicellularity has also been independently invented in at least 20 radiations since the Cambrian concept took hold in the animal radiation. Most familiar are the land plants, first showing up as moss-like organisms ~400 MYA, whose mode of multicellular organization and embryogenesis is totally different from the animal version. The fungi (think mushrooms) do it in yet another fashion. The green algae in the Volvox lineage have come up with yet another set of ideas.

So why bother with multicellular selves at all when unicellular selves obviously do just fine?

One answer is that it has to do with predation: that larger forms are harder to eat. Another answer is that is has to do with cooperation: a group of cells that are programmed to live in proximity are poised to supply each other with nutrients and to differentiate specialized features, adaptations that promote continuation of their kind. Indeed, it is becoming increasingly clear that many kinds of bacteria, while remaining single-celled, have evolved sophisticated systems of communication with one another.

From the human perspective, perhaps the most significant consequence of our animal mode of multicellularity is that it heralded the onset of an evolutionary trajectory that made possible both our mortality and our minds. I’ve riffed on this story in a previous blog, but the punch lines bear repeating. In the animal lineage, one set of cells, called the germ line, is entrusted with passing on genetic instructions to the next generation as eggs and sperm. This allows the rest of the cells in the embryo to differentiate into the ~100 different cell types that comprise the newborn soma, where the soma will eventually age and die. All 10 trillion cells in the soma have the same “goal”: to cooperate such that the organism flourishes and hence reproduces via its germ line. They also cooperate with the 100 trillion bacterial symbionts that provide important additional traits. But their goal is not immortality: immortality has been handed off to the germ line.

The news stories about the Gabon fossils focused on the fact that the notion that multicellularity began with the Cambrian Explosion was off the mark by some 2 billion years. But the deeper message is that the unicellular vs. multicellular option has been with us from the get-go, playing itself out in independent, diverse, and fascinating trajectories.

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