3 Domains of Life

What is the most diversity you can imagine in nature?   We usually think of insects, animals, and perhaps plants (shown in red).  But what of prokaryotes?  They are amongst the most diverse creatures, but not necessarily from outward appearances. Some people use the term “prokaryote” to refer to bacteria, but that’s not quite right.  From a biochemical perspective – which encompasses what they are made from, what they can eat, where they can live and many other features – the “prokaryotes” are quite diverse.  In fact, the shortcut term “prokaryote” groups together two fairly dissimilar branches from the “tree of life”:  the Bacteria (aka Eubacteria or “true bacteria”. shown in blue) and the Archaea (shown in green).

From a longevity perspective, the prokaryote group has remained in the same form for the longest:  they have “primitive features”, but are as fully modern as any other organism you might find in the world today.  From an evolutionary perspective, they are “successful”.  They have the largest impact on our atmosphere and all ecosystems – bar none.  Plants make some oxygen, but most is released from photosynthetic bacteria.  Nutrients are made bioavailable from leaf litter and even from the nitrogen in our atmosphere.  Atmospheric nitrogen is not very reactive:  legumes form really cool bioreactors that are anoxic (no oxygen, which would undo the biochemistry) in their root nodules.  Their root cells make molecules rather like hemoglobin to scrub away the oxygen that surrounds the nodules to ensure most efficient fixation of the nitrogen (the process of taking gaseous nitrogen and converting it to nitrogen-containing chemicals used by organisms).  But although the plants (eukaryotes) create a living laboratory, it’s up to microorganisms to do the actual chemistry.  These diazotrophs could be bacteria or archaea, but are usually specific to the roots in which they work.

But back to the question:  diversity.  How do we measure it?  We could say that a worm is simpler than an alligator.  But inside, they are very, very similar.  A mouth, a gut, an anus.  Nerves.  Muscles.  Collagen in their skins (and, if they have them, bones).  Not so different in the end.  But bacteria are just dots, right?  Under a microscope, you might use different coloured stains to bring up their features.  In fact, that’s not enough.  Microbiologists sometimes use special media to figure out the biochemical basis of unknown strains.

The point is that diversity contributes to the complex interactions that create and support our biosphere.  There’s a lot of biochemistry going on, and it’s not done by any one organism.  If it takes a village to raise a child, it takes a whole ecosystem of organisms to generate a living world!  The diversity involved is not simply how an organism looks … but also how it does its biochemistry.

Although he probably wasn’t thinking of ecosystems, Heraclitus (c.535 BC – 475 BC) suggested “…confess … all things are one”.  And it’s true.  Every living organism is related by descent from a Universal Last Common Ancestor that had proteins, lipids, and sugars as their body plan, and with genetic instructions imprinted in their DNA.  Change in that DNA over generations – but the stability to keep traits mostly constant – is what provides us with our diversity.