The Major Classifications of Life

by Deb on October 13, 2009

Everyone’s heard of plants and animals, right?  Well, there’s actually a lot more to life than that.  What you can see around you is a miniscule fraction of life on earth.

With the discovery of lots of microscopic life Plants andAnimals got a bit uncomfortable and life was divided into the Eukaryotes and Prokaryotes.  Fancy names, they are the cells that have nuclei and cells that don’t have nuclei.  In Eukaryotes, which we are, the inside of the cell has lots of distinct areas called organelles, almost like rooms or different factories.  Prokaryotes, the bacteria and blue-green algae, don’t have organelles but everything floats around inside them.  Nuclei, which hold all the chromosomes together, made sexual reproduction work well, which leads to lots of sorting and recombining of genes, which lead to all the different forms of life we are most familiar with.  But don’t think the Prokaryotes are somehow not as successful just because we don’t see them – in terms of just about anything you care to think of, (mass, volume, number of species, number of individuals) it is now, and forever will be, the Age of Bacteria.  But, well, they’re bacteria.


About 30 years ago people started studying the methanogens, ‘bacteria’ that don’t use oxygen for energy but produce methane.  They are the basis of a lot of the deep sea communities that don’t use sunlight, but are also found in places like the gut (that’s where all that methane cows and sheep produce comes from).  It was discovered that they were actually very different to bacteria, and were put into their own domain called ‘Archaea,’ because it was thought they were very old remnants.  However it is now becoming clear that they are much more common than was thought and have some characteristics like bacteria, some like eukaryotes, and some that are unique.  So now we have the 3 domains of life – Bacteria, Archaea and Eukaryota.  These are three completely different ways of building cells, making energy, and doing all the reactions that power life.

Eukaryote cells have organelles and are a lot larger than bacteria and archaeans.  In fact, some of our organelles look remarkably like bacteria living inside our cells – mitochondria (the part of our cell that makes energy) even has its own DNA.  So it seems that at some point in the dim dark past parasite bacteria and their hosts became so necessary for each other that they became a new type of life.  They’re certainly not bacteria any more – they are an integral part of our cells, a bit like an appendix is no longer used to digest leaves but we can still guess that’s what it did.

Within the Eukaryotes there are 4 Kingdoms – Protista, Animalia, Fungi and PlantaeProtists are the single-celled creatures – paramecium, amoeba, euglena.  Remember peering down a microscope in Year 8 at a bit of pondwater?  They come in a startling array of shapes and sizes, moving like oozing blobs, using whips, or beating hairs around the outside.  Some have chlorophyll like plants, some engulf bits of matter, other cells or bacteria.  Even though they are all made of just one cell,they are more similar to us than they are to bacteria or archaeans because they build and reproduce their cell the same way we do and use the same chemical reactions to power themselves.


Sometimes protists form colonies, and this is a clue to how multicellular life may have started.   A sponge is an animal, but you can chop it up, even put it through a sieve, and it will go back to being a sponge.  The other possibility is that a large cell grew internal divisions to end up with many cells.  It’s probable that both happened, because it happened several times.  Just because all animals get their energy and nutrients in the same way doesn’t mean we all descended from one original animal, there are at least three different lineages.

Animals get their energy by using oxygen to burn sugar, which they get by eating.  That’s how they get their other nutrients and water too.  They are usually mobile at some point in their life cycle otherwise they would run out of food, and usually have some form of sexual reproduction.  Don’t be fooled by a few large mammals, we are actually one of the largest animals.


Plants make their own energy by using sunlight and green chlorophyll to react water and carbon dioxide to make sugar.  They then use oxygen to burn the sugar and release that energy.  They are generally only mobile as seeds then use roots to collect nutrients and water from the ground.  They reproduce in many different ways, including sexually or through budding, and a complex such as a grass can grow enormous.  So is a lawn one plant or many?


Fungi can be rooted and do not eat, like plants.  But they don’t make their own energy either.  Instead they are decomposers – they break down other tissues for their nutrients and energy.  They have extremely complex reproduction, with both asexual and sexual cycles.  Some of their spores can lie dormant for many years and survive extreme conditions.

Mushrooms on Moss

So there you have the major divisions of life, the work of centuries in less than 1000 words.  But I’ll leave you with one more thing to ponder – try Googling “Are viruses alive?” and skim through the first page of the search results.

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