Hey it’s Professor Dave, I wanna tell you about the tree of life.
By now we know a lot about natural selection and the ways that populations of organisms
can change dramatically over long periods of time, just like slow geological processes
that create mountains and canyons.
Over the half-billion years since the Cambrian explosion, an unbelievable array of organisms
has come to be, most of which are gone, but plenty of which are still here.
We can explore the world and find strange creatures in the jungle and at the bottom
of the ocean, and we can look at fossils to see what once was.
So how do we keep track of all these different kinds of life?
Which creatures are more closely related, and which are more distant?
To stay organized, we use something called the tree of life.
We can think of the trunk of this tree as the unicellular common ancestor of every living
thing in history.
Of course this is long extinct, but it produced other organisms, which are represented by
different branches, and these in turn produced other organisms, and so on, until millions
of years later, here we are, along with whales and birds and porcupines and bears.
The process by which one species produces a new species is called speciation, and this
is the bridge between microevolution, which looks at changes in allele frequencies, and
macroevolution, which is a broad pattern that produces new organisms that look totally different
from pre-existing ones.
We have a way of classifying an organism that tells us exactly how it is related to everything
else, how recently or distantly, and every organism belongs to a particular species.
These are groups of populations that have sufficiently similar genetic material such
that they are capable of breeding to generate viable, fertile offspring.
If two organisms can do this, they are of the same species.
If not, they aren’t.
But before we start with the trunk of the tree of life, let’s quickly recall exactly
what happened in the early history of life on earth.
We’ve already talked about the origin of life, but let’s review a few points to put
things in perspective.
From the famous Miller-Urey experiment we saw how conditions on the early earth, just
after it cooled enough for liquid water to exist, were such that amino acids and nitrogenous
bases could form spontaneously.
These were able to polymerize to form long chains, and when some of these were by chance
encapsulated in a self-generating membrane, like the way soap micelles form, the first
proto-cell was born.
Once nucleic acids became self-replicating, the concept of inheritance took hold.
We also discussed how multicellular life arose from unicellular life, which resulted in an
even greater variety of organisms.
This is admittedly a speculative series of events, and one could go into much greater
detail on any individual step, but it is rooted in firm science and testable hypotheses, so
let’s move to the time period where an extensive fossil record begins.
We can look at fossils to learn about living organisms that existed at any point over the
past several billion years, which gives us incredible insight into the history of the earth.
We divide this history into different eons, beginning with the Hadean eon, where the earth
was still a hot ball of molten rock.
In the Archaean eon, prokaryotic life arose, living alone for a billion and a half years.
This is when the earth became covered in oxygen, as the product of photosynthesis.
In the Proterozoic eon, eukaryotic life evolved through endosymbiosis, and eventually multicellular
eukaryotic life, through cell specialization.
This eon ends with the Cambrian explosion.
And lastly, the Phanerozoic eon starts a little after animals show up on the scene, and is
divided into the Paleozoic, Mesozoic, and Cenozoic eras.
This whole eon is around half a billion years long, starting just before animals make it
onto dry land.
Dinosaurs dominate the Mesozoic era, and humans show up only in the last sliver of the Cenozoic era.
The boundaries between these eras and eons mark huge extinction events that can be seen
in the fossil record, and we continue to examine fossils to learn about life from all periods
of biological history.
Now that we are up to speed with the history of life, we want to know about the phylogeny
of various species.
This is their evolutionary history, or ancestry, and the discipline of taxonomy is what we
use to classify every species in the context of its ancestry.
For greater context, we look at the tree of life.
This tree has many branches, where different species evolved from a common ancestor.
The more recently two species converge as you go down, the more recent their common
history, and the more closely related they are.
As you continue to go down the tree, species become less and less related, so humans and
chimpanzees, whose branches are very close to one another, diverged from a common ancestor
only a couple million years ago, whereas humans and jellyfish are much more distantly related,
having diverged hundreds of millions of years ago, when all life was in the sea.
If we get all the way to the bottom, the trunk of this tree marks the beginning of life on
earth, a single-celled organism that everything alive today is descended from.
Since we don’t know precisely what that was, we start with what we know, and at the
most basic level, we consider three domains of life.
These are bacteria, archaea, and eukarya.
Bacteria and archaea are the unicellular prokaryotes we talked about, and eukarya is all eukaryotes.
This means unicellular protists as well as all multicellular life on earth, including
every plant and animal there is.
Within the domain eukarya, there are several kingdoms.
Animalia contains all animals, plantae contains all plants, fungi contains all mushrooms and
molds, and protista contains all the rest, though many biologists don’t recognize this
as a valid kingdom any longer.
Within each kingdom there are several phyla.
Within the animal kingdom there are nine major ones.
There is the phylum arthropoda, which contains all kinds of bugs and lobsters and creepy
crawlies, mollusca, which contains octopi and snails and other such things, and most
notably there is chordata.
This phylum includes any animal with a spinal cord, and the divergence of vertebrates and
invertebrates was a major step in evolution.
Within each phylum we can find different classes of organisms.
Chordata contains the class reptilia, which are reptiles like crocs and snakes and lizards;
amphibia, which are amphibians like toads and frogs; and mammalia, which are all the
mammals, from kangaroos to elephants.
Humans belong to mammalia, and within this class there a number of orders.
The order cetacea contains all the whales and dolphins.
Carnivora contains things like bears and weasels.
Then there are the primates.
That’s got everything from orangutans to lemurs, and humans are in there too.
Within the order of primates, there are several families.
The family hominidae contains all the great apes, so that’s us again.
Within hominidae every organism belongs to a particular genus, and that’s where we
diverge from the apes.
Our genus is called homo, which means man in latin.
Within a genus there are several species, and all the species within the genus homo
are now extinct except for homo sapiens, and that’s us.
The species that came before us, homo erectus, homo habilis, and many others, are all gone,
and homo sapiens, which means wise man, sits at the very top of the tree, the only species
to have evolved an intelligence that spawned complex civilizations and information storage,
first as written language and eventually the omnipresent internet, which you’re using
right now.
So every species can be classified, and there are mnemonic devices that can help us remember
the different tiers of categorization.
Do kangaroos prefer cake or frosting, generally speaking?
Or, did King Phillip come over from great Spain?
You can even make up your own.
To review, we humans are collectively the species homo sapiens.
We belong to the genus homo, the family hominidae, the order primates, the class mammalia, the
phylum chordata, the kingdom animalia, and the domain eukarya.
We can trace a path from the tip of our branch all the way to the trunk, and every organism
on earth will converge with our path at some point, whether very recent, or very distant,
which means that every living thing is related in some way.
To see precisely how related two species are, we could use sophisticated techniques like
ribosomal RNA sequencing, or we can simply examine homologies.
These are similarities between organisms, be they genotypic or phenotypic, and they
typically suggest common ancestry, as an organism’s evolutionary history is documented in its genome.
But to get into these details for specific animals, we will have to wait until the zoology
course, and a discussion of everything that’s inside a human will have to wait for the anatomy
and physiology course, both of which will come in the future.
For now, let’s move forward and wrap things up with a quick survey of biotechnology, so
we can see what humans have been able to do with all of this powerful biological knowledge.