How many ways are there to be multicellular on Earth? You know, organisms made of more than one cell? Let's see . . . plants, animals, and fungi. Three, right? Wrong.
I give you "Representative Diverse Origins of Multicellularity ...", aka, Fig. 1 from the paper "The Evolutionary-Developmental Origins of Multicellularity" in the January issue of the American Journal of Botany. I should emphasize this figure only includes eukaryotes -- organisms whose cells contain DNA-storage compartments called nuclei -- and not the bacteria, which have also evolved multicellularity several times.
I love looking at figures like this. It gives me the feeling that there's a whole world of fascinating life forms out there that I still, even after all these years studying biology, have yet to glimpse. And there's a reason for that: it's true! The even better news: we don't even have to leave Earth to see them. They're right here, just waiting for you to notice them.
Biologists don't actually agree on what it means to be a multicellular organism, which you might define simply as being an organism made up of more than one cell. But as with many things biological, in the real world, there is a lot of gray area. There's actually quite a lot of fundamental concepts biologists don't agree on (want to start a fight? Ask two biologists to agree to a definition of "species"), which is why it's so ironic that the significance of evolution by natural selection is one of the few things they do.
In any case, depending on how loose or stringent the definition, multicellular life has evolved somewhere between 13 and 25 times on Earth, including at least three times among the fungi, and twice each among the red algae, stramenopiles, and chlorophyceans, which all appear to be a single occurrence above. Also, the figure above by no means represents all eukaryotes. Many have been left out. Still, this image gives you a nice feel for the spectrum of multicellular diversity, and the fact that multicellular life has evolved over and over and over again -- which means it must confer some significant advantage!
Here's a quick cheat sheet to the more impenetrable names on this figure:
- Charophycean algae -- A group of plant-like green algae. Some are called "stoneworts" and resemble the early-evolved plants called horsetails, others in the Coleochaetales are cute little clusters of branched cells that live lives of quiet dignity on the underside of water plant leaves. Many scientists think land plants evolved from a now-extinct member of this group.
- Chlorophycean algae -- The rest of the green algae, including everything from unicellular Chlamydomonas (which makes pink snow, as recounted here last year), to the famous spherical colonies of Volvox, in which 500 to 60,000 cells resembling Chlamydomonas unite to form an empty ball. Since only a few cells can reproduce (i.e., the cells are specialized), many consider it a multicellular organism.
- Dictyostelid slime molds -- Bizarre organisms in which thousands of amorphous single-celled organisms called amoebae, some closely related, some not, aggregate into a giant (for ameobae) slimy slug that crawls toward a suitable spot to release its spores. Where do the spores come from? When the slug finds its spot, it transforms itself from a slug into a stalked fruiting body, in which 20% of the amoebae selflessly sacrifice themselves to form the stalk. The rest metamorphose into spores; some cheat to avoid becoming stalk-fodder. Before aggregation to the slug, individual amoebas may farm bacteria.
- Plasmodial slime molds -- These are similar to dictyostelid slime molds in that they evolved from the amoebas. But unlike the dictyostelids, two amoebae mate and then this single organism duplicates its own nuclei over and over again to produce a giant crawling bag of nucleus-filled cytoplasm. They can get so big they have been mistaken for piles of dog vomit, but most are tiny and produce jewel-like fruiting bodies of exquisite beauty, like the one you can see in the background of my twitter profile (and my original blog's masthead).
- Metazoans -- This means you. Metazoans did not rate their own little descriptive cartoon icon in this figure. I nominate this guy for the job.
- Fungi -- A fascinating, beautiful and overlooked kingdom of organisms with external digestion and walls made of chitin (not cellulose, like plants and houses made of plants (you might live in one of these)). Though many are decayers, nearly every plant has several fungal partners living on or in its roots and inside its body. Without them, terrestrial plant life might never have advanced beyond the Slimy Green Film stage. Beware if you are a nematode worm.
- Red algae -- Reddish brown seaweeds with often-Byzantine life cycles. You have likely seen these if you've spent much time around the ocean.
- Ciliates -- As the name implies, these are usually bacteria-eating single cells covered in tiny beating hairs called cilia that constitute the propulsion system. But a few ciliates can form slime-mold-like aggregations that produce stalks bearing spore-like propagules. Pictures (courtesy Psi Wavefunction) here.
- "Other Stramenopiles" -- This group includes miscellaneous seaweeds, including brown algae. You know those giant kelp forests hundreds of feet tall off the coast of California that sea otters love frolicking in in all the nature documentaries? Brown algae. Also in the group: golden algae, yellow-green algae, and the fungus-like oomycetes(oh-oh-my-seats), also called "water molds". An oomycete called Phytophthora infestans caused the Irish potato famine, not a fungus.
- Acrasid slime molds -- Similar to Dictyostelid slime molds, but evolved from a whole different non-amoeba branch of the family tree.
In the article accompanying this figure, the author, Karl Niklas (my college botany professor, as noted in the caption above) thinks Deep Biology Thoughts about why multicellularity has evolved so many times -- at least once in every major group of eukaryotes -- what the requirements of multicellularity are, and the many different ways life has taken to get there. The predominant route taken by algae, land plants, fungi, and animals seems to be unicellular --> colonial (loose federations of cells) --> multicellular. But an alternate path involving a network of tubes or a giant cell with many intermingling nuclei inside ("siphonous/coenocytic --> multicellular") seems to exist that has been taken by fungi, those pesky oomycetes, and some algae.
He also notes, among many other observations, that the various multicellular lifeforms appear to have achieved the same ends by many means. For instance, all multicellular life may possess cellular junctions -- basically, doors between cells -- that help the cells in a single organism talk to each other and coordinate their activities. But the particular chemicals and structures that make up these junctions have many different origins. The same could be said for the structures called "leaves" in plants but called laminae or blades by algal biologists, phyllids by moss biologists, and fronds by fern biologists. Though they appear similar and have similar functions, they evolved separately and differ structurally. In other words, Niklas says, evolution appears to have acted on different genes and gene networks in ancient unicellular organisms to achieve the same functional result among many distantly related groups.
Niklas K.J. (2014). The evolutionary-developmental origins of multicellularity, American Journal of Botany, 101 (1) 6-25. DOI: 10.3732/ajb.1300314