This information is designed for people who have not previously come across the concept of the Tree of Life.
Think of your own family. A family tree can be drawn showing how everyone is related. If you travel backwards in time along this tree you will meet parents, grandparents, great grandparents and so on, from whom you are descended. However, you may also be able to follow the links sideways and locate your parents' brothers and/or sisters - - and if you travel forwards in time again you may locate their children - your cousins.
Everyone belongs in a family tree but if you go back far enough, every family tree joins together. There are no exceptions. This means all humans are related. You may already know that our relatedness can be measured in our genes and DNA. Using this science, relatives separated at birth can be proved to be brothers or cousins etc. Indeed we pass on genes (made from DNA) whenever we reproduce. This DNA contains the code required to build our bodies and minds. However, unless you are an identical twin (when you share identical DNA) relatives don't look completely alike because the parents' DNA in the egg and sperm are mixed up creating new combinations. However, because family members are so closely related, their likenesses are immediately obvious. Further, occasionally, DNA copying errors occur to create new DNA sequences we call mutations. Most of these are bad for us and may cause genetic diseases or worse, but some are, by chance, actually even better than the original.
Charles Darwin and Alfred Russel Wallace were the first to suggest how, over long periods of time many, many such tiny changes accumulate to produce much larger changes (and new species) in a process we call evolution. The lifeforms with the 'best' DNA combinations were more likely to survive and reproduce, thus passing on these 'fittest' genes. Effectively nature is steadily, if unwittingly, 'selecting for survival' only life forms whose features allow them to compete at least as well as the rest in that habitat. We call this natural selection. Over time you end up with many different species all well adapted for the habitat in which they evolved. Any individuals less good at surviving or breeding, don't make it.
A short philosphical digression : This isn't cruel, there is no intention to be nasty, in fact the process has no intentions at all. It just works that way. We see it as hard and cruel only because we have the most highly developed empathy on the planet. But just like every other species, our ancestors originated through this process and in darker moments it is not uncommon to hear people talking about us being 'just' animals underneath, cruel and hardened for survival over millions of years. Survival has been a struggle for sure, but there is no need for the adjective 'cruel'. Only a tiny minority of humans are intentionally cruel - we classify them as mentally ill or recognise they have been cruelly treated themselves, or a combination of the two.
Most of us are exceptionally compassionate and nurturing - far more so than our cousins in the rest of nature.
And even for animals it's unfair to describe them as 'cruel'. I once found my 6 month old daughter pulling the legs off a daddy longlegs. I stopped her but it was obvious she wasn't being intentionally cruel. 23 years later she's about to start work as a radiotherapist and like many other humans, hasn't got a cruel bone in her body.
Most of us also understand that animals are equally innocent in their actions - incapable of empathy but certainly not intentionally cruel. For most adults empathy is so highly tuned that we assume others - even animals -must be acting intentionally when we see suffering. It hurts us to see this. We recoil and sometimes despair in our own nature.
We need to be kinder to ourselves here. If you've ever had the misfortune to have someone close become seriously ill you'll likely have received a huge outpouring of compassion from friends and family - even strangers. There is no doubt humans can be are incredibly compassionate and phenomenally co-operative. It has been argued that our success as a species is primarily down to our capacity to intelligently empathise and collaborate.
Anyway, back to evolution. Next we are going to relate this to the Tree of Life. Let's keep going backwards in time to meet our ancestors. Great, great grandparents, and then great, great, great grandparents - but now, instead of writing 4 'greats' in front of grandparents, let's write 250,000 'greats'. Given that humans are on average around 20 years old before they have children, that means we have travelled back around 5 million years. When those grandparents were alive, what would they look like?
Grandma, what thick body hair you have, and Grandpa, you look very at home up that tree! We are directly descended from these creatures but they are definitely not humans. Nor are they chimps, gorillas or monkeys as we know them today. We humans are called the 'Naked Ape' because we are apes who have largely lost our hair. But back then neither humans nor chimps had evolved yet, and instead these distant hairy ancestors would be the nearest we would find to both humans and chimps.
This is where the Tree of Life concept takes over from the family tree idea. You are related to your cousins but clearly you did not come from them. You came from your parents and grandparents. Your cousins simply share one set of grandparents with you - you both 'share' the same ancestor. Now let's return to your 250,000th grandparent above. You and I came from that ancient ape, but this is the amazing insight science tells us, SO DID THE CHIMPS. You can think of one of its babies growing up to have children that themselves had children and so on until you reach the human race. Equally you can think of its sister or brother growing up to have children, that had children, until we reach all chimps. Looking at it this way, we can think of chimps as being our distant cousins.
We can now imagine two lines, twigs in a tree if you like. At the tip of one is all humans together, the species known as Homo sapiens, and at the tip of the other, all chimps. If you travel back in time you will simultaneously travel down each twig until they meet. That meeting point is the ancestor we share with chimps that lived around 5 or 6 million years ago.
The process of evolution I described earlier gave rise to the differences we see between humans and chimps today. This video by 'Stated clearly' explains evolution rather nicely.
Going back to our branching tree idea, if we go even further back in time, the two now joined twigs will carry on as a combined stem. As we travel down this stem we continue to go backwards in time and at around 8 million years ago this stem merges with another twig. This twig represents another set of even more distant cousins. If we switch time direction again, this ancestor gave birth to babies, one of which went on to become the ancestors of all humans AND chimps, the other became gorillas. In this way gorillas are also our cousins, if a little more distant.
As we continue back in time these shared 'common' ancestors re-unite us with more and more branches and our twig becomes part of a tree including orang-utans, monkeys, all mammals, birds and reptiles, amphibians, fish, sharks, starfish, insects, crabs, worms, shellfish, jellyfish, sponges, amoeba, plants, algae and bacteria, until eventually we get to the bottom of the trunk of the tree. We are very, very distant cousins with every branch, in fact with every living thing on earth - without exception. Now we can see the whole tree of life including every creature alive today and, strictly, every creature that has ever lived, including all the species now extinct like the dinosaurs - all cousins together in one huge family tree.
Darwin's famous sketchbook scribble of the Tree of Life 1837.
At this point we have travelled around 4 billion years back in time and traced our roots to a tiny single celled 'grandparent' that most closely resembles a bacteria. We will have reached the origins of life, a profound moment indeed, and we think worthy of a celebration. If you walk the human trail of the Ancestor's trail you will follow our line of descent to this point. Those walking the gazelles trail, for instance, will also end up at the same place, but once the two trails merge, people would carry on together along the same trail. In this way our ever increasing band of pilgrims arrive together at the origins of life.
Don't forget there is nothing special about the human trail, every species has its own pathway through the tree. We just happen to be, as far as we know, the only species that has figured this out.
For more visual learners David Attenborough explains the Tree of Life
If you want to explore the tree of life for yourself online take a look at the amazing ONE ZOOM.
Here are some example visual representations of the tree of life.