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We are the Tail that Wags the Dog: Bacteria as the Dominant Life Form on Earth

We are the Tail that Wags the Dog: Bacteria as the Dominant Life Form on Earth

Sometimes, those of us working in the biological sciences might just wonder why we’re putting all that time and effort into studying whatever it is we study.

For those of us working with bacteria, our justification doesn’t have to go much beyond the fact that they are the dominant life form on the planet. They always have been and no doubt they always will be.

Some reasoning behind the claim for bacterial dominance was nicely laid out by the late paleontologist and super essayist Stephen Jay Gould (2).  I’ll sum up some of his criteria here, along with a few more recent findings.


Bacteria are the first organisms that appear in the fossil record. The oldest examples date from about 3.5 billion years ago. In contrast, eukaryotes arose only about 1.8 billion years ago.

So, bacteria were the only form of life for most of earth’s biological history, and they’re still the dominant form (more on this below).


Bacteria are unmatched in numbers, variety, environments inhabited and modes of metabolism.  It’s difficult to imagine a global catastrophe that would wipe them out (like cockroaches, but they’re so much nicer).


Of the three evolutionary domains, two consist entirely of prokaryotes, namely Bacteria and Archaea.  Members of these two domains differ in fundamental ways.  In fact, the evolutionary diversity between prokaryote subdivisions is as great as that between the major kingdoms of the Eukaryote domain, like plants and animals.


Bacteria as a whole tolerate a big range of temperatures, pressures and pHs.  They are found just about everywhere, from the earth’s deep subsurface (5) to within and on our bodies (4).

They also have metabolic capabilities far greater than those of other organisms, which is why, for example, we don’t see a lot of biological polymers like cellulose or chitin accumulating in the environment (1).

Numbers and Mass

Because most bacteria can’t be grown in culture, we’ve learned much more about their previously hidden presence and diversity thanks to advances in genomic sciences like PCR and nucleotide sequencing technologies.

The number of bacterial species probably reaches into the millions (3) and the estimated number of individual bacterial cells on earth is 4-6 x 1030.  (5).  In comparison, there are “only” about 2-4 x 1011 stars in our galaxy.   The biomass of bacteria is estimated to be about 4.5 x 1017 g carbon, which is similar to the biomass of all plants (5).

Here are some other really interesting and more personal stats

Our intestines contain about 1014 bacterial cells, which is an order of magnitude more than the number of human cells in our bodies.  This “microbiota” has an enormous influence on how we develop and function: on balance, they’re good for us.

Collectively, our microbiotal genomes (microbiomes) have maybe 5 million genes, which outnumbers our human genetic complement by about one hundred fold.

Our human component plus our microbiome do indeed equal a “superorganism” (4)!

To wrap up, bacteria do much for us, and I’m glad they’re here.  In the future, they will continue to serve us (in biotechnological processes, for example), and we’ll sure continue to serve them as long as we’re around.


1.  Beier, S., and S. Bertilsson. 2013. Bacterial chitin degradation- mechanisms and ecophysiological strategies. Frontiers Microbiol. DOI:10.3389/fmicb.2013.00149.

2.  Gould, S. J. 1996.  Full House: The Spread of Excellence from Plato to Darwin. Three Rivers Press, NY.

3.  Rinke, C., P. Schwientek, A. Sczryba, N. N. Ivanova, et al. 2013. Insights into the phylogeny and coding potential of microbial dark matter. Nature DOI:10.1038/nature12352.

4.  Sommer F., and F. Bäckhed. 2013. The gut microbiota- masters of host development and physiology. Nat. Rev. Microbiol. 11: 227-238.

5.  Whitman, W. B., D. C. Coleman and W. J. Wiebe. 1998. Prokaryotes: The unseen majority. Proc. Natl. Acad. Sci. USA 95: 6578-6583.

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