The transition (during evolution) — from single cells to multicellular life

From time to time, these GEITP pages examine EVOLUTION –– especially from the standpoint of “how do an organism’s genes respond to the adverse environment at that moment in time, and mutations/insertions/deletions/inversions/duplications (i.e. DNA-sequence alterations) occur, as well as epigenetic changes (i.e. no DNA-sequence alterations) occur, in order to survive and perhaps diversify into a “new, improved organism” with even greater likelihood to survive?”

In order for everyone to “start on the same page,” let’s give a brief history of Earth and life on Earth. The planet formed ~4.54 billion years ago (BYA), and earliest forms of life (single cells) are believed to have originated between ~4.2 BYA and ~3.5 BYA (give or take a few months). The oldest fossils of single-celled organisms identified to date lived ~3.5 BYA. Fossilized microbes in rocks of Western Australia are claimed to have been alive ~3.4 BYA. Two main groups of (single-celled) life –– bacteria and archaea –– had diverged from one another, some time between ~4.0 BYA and ~3.0 BYA; viruses probably predated the time of this divergence.

For reasons still debated, between ~2.4 BYA and ~2.1 BYA atmospheric O2 increased dramatically on Earth. Around ~2.0 BYA eukaryotic cells (containing a nucleus plus organelles) first emerged, and then diversified ~1.5 BYA into the three major lineages: Plant (cellulose-containing cell wall, nucleus, chloroplast organelles with chlorophyll, participating in photosynthesis, i.e. CO2 uptake and O2 output) Fungus (chitin-containing cell wall, nucleus, no photosynthesis, but taking up nutrients via absorption), and Animal (cell membranes, nucleus, organelles including mitochondria, which create energy for the cell by O2 uptake and CO2 excretion). Earliest multicellular plants (e.g. slime molds) and animals (e.g. sponges) originated ~0.8 BYA (i.e. ~800 million years ago).

The transition from unicellular life to multicellular life –– was believed to be a momentous step forward in evolution. Single cells had a modest existence: simply feed yourself. In contrast, cells in a multicellular organism, from the four cells in some algae to the 37 trillion cells in a human, give up their independence to stick together and take on specialized functions (e.g. finding food, reproducing, and surviving predators).

Recently we have begun to understand more about this “transition from unicellular life to multicellular life” [see attached 4-page editorial]. Genomic comparisons between single-celled and multicellular organisms have shown that most of the genes were likely to have been in place well before multicellularity evolved. Experiments have now shown that single-celled life can evolve into the beginnings of multicellularity –– in just a few hundred generations — which, in terms of evolution, is less than a nanosecond. Plants and animals each made the leap to multicellularity just once. However, in fungi, the transition took place repeatedly.

All told, by surveying active protein-coding genes in 21 choanoflagellate species, authors discovered that these “simple” organisms have ~350 gene families once thought to have been exclusive to multicellular organisms. The ancestral versions of those genes, in many cases, do not perform the same functions, as compared with functions in multicellular organisms. For example, choanoflagellates have genes encoding proteins that are crucial to neurons, yet their cells do not at all resemble nerve cells. Likewise, their flagellum (tail) has a protein that, in vertebrates, helps create the body’s left-right asymmetry. What this “neuronal” protein does in the choanoflagellate is not yet known. In conclusion, these studies have shown that many functions of specialized cells in a complex organism are not new, but rather is derived from the same gene, found in single-celled organisms, that has taken on a new function. The term for this is convergent evolution.


Science 29 Jun 2o18; 360: 1388–1391 [editorial]

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