The first cell, and you
Some time roughly 4 billion years ago, while the Earth was young enough that club bouncers still asked it for ID, something very very strange happened. A random mixture of lifeless matter, probably gathered around a hydrothermal vent, stopped being lifeless. Somehow, a tiny bubble of goop worked out a way to make more bubbles of goop, using instructions that allowed the new bubbles of goop to make yet more bubbles of goop. The first living cell had formed, and quickly got to work making more of itself. How this happened is a story for another time, not least because nobody actually knows.
It may seem incredibly unlikely that something as complex as life could form from none-life, but it’s worth bearing in mind that, in a universe that is billions of years old and billions of light years across, even the unlikely things are virtually inevitable.
Perhaps the most fascinating thing about that cell is that it not only became alive, but that it never died. It is still alive today, in a sense. Rather than dying, it divided into two daughters, who divided into two daughters, and on and on, until it spread into new ecosystems and environments, and the crafty hands of evolution shaped it into countless new species. Every living cell on Earth today can trace its ancestry in a direct, unbroken line to that first cell, without a singe cell dying along that entire lineage. Because of course, dead cells have no daughters.
So within you, within the very brain constructing the thoughts that allow you to read this text, are cells that have carried that spark of life for over 4 billion years, undying and ageless, albeit much changed, and with many other branches leading far away to other regions of the tree of life.
One branch on the tree of life
With somewhere between 10 million and 1 trillion living species in the world today (yes that really is the margin of error we’re working with), the tree of life has become rather crowded. Showing all those millions of branches of the tree would be a gargantuan task, and downloading an infographic that big would take weeks. So instead, I’ve taken a very lazy and human-centric approach: I’m only showing the times other organisms have branched from us (or more accurately our ancestors). Actually I’m being even lazier than that: I’m only showing every time a branch-off resulted in a clade (group of related organisms) that is still around today. All branches that died out entirely, are ignored.
Imagine tracing one of your cells backward through time. Watch every cell in your body rewind and gather back into the fertilized egg that made you. Keep going back through your human ancestors, and then further still. You watch your ancestors’ faces grow hairier, then pointier, then scalier. Soon, there’s no face at all, and eventually just a single cell. And still, you go back, all the way to the first ever cell. As you go, you make a note only when our ancestors split from other lineages that are still alive today.
The left of this image shows our lineage, while the right shows all those other pathways, the evolutionary roads that your ancestral cell didn’t take, but some other cell did, to great success.
It’s worth pointing out here that humans aren’t special in this regard. I could have picked any organism on Earth and made an image like this one, and some would likely have been even bigger and more complicated. If we take a step back we will realise that we are not the pinnacle of evolution any more than magpies or reindeer are: I don’t want this image to encourage the idea that evolution is a ladder that we climbed, with humanity at the top. We are just one tiny branch of a vast and complex tree of life. I picked humans for this mainly because I am one, as I suspect are most of my target audience.
Reading this image
For each fork in the tree of life I’ve included some information:
On the left is the taxonomic rank. This is the name of the group that includes all organisms that split off after this point. The order humans belong to, for example, is the primate order. All primates descend from a single ancestor that lived around 66 billion years ago, around the same time the majority of the dinosaurs (and every land animal larger than a Labrador retriever) were absolutely wrecked by an asteroid impact.
To the right of this is an arrow, showing the earliest (surviving) group that split off after that first primate. On this arrow is a number that represents roughly how long ago this branch split off, in millions of years. The margin of error on these estimates gets increasingly huge as we go back in time, until, beyond the animal level, I stopped bothering with them entirely. Right of this we have the name of the split-off branch, which in the case of primates, is Strepsirrhini, a suborder of primates that includes lemurs, lorises and pottos.
A small number in the right of this label shows roughly the number of living species in this clade. Early on this is fairly easy, as mammals tend to be pretty big and well documented creatures. Further down the image these numbers get increasingly rough, often being vague estimates, or simply showing the number of species we have recorded so far, with many more likely being out there. So take these numbers with a huge pinch of salt.
And finally on the right of the image, I’ve done a little picture to show an example or two of an organism in this branch.
Theoretically, these branches leading off to the right should include every living thing on Earth, from our closest relatives (chimps and bonobos) branching off at the top, to our most distant relatives (bacteria) branching off right down at the bottom, not long after life began. And at the bottom, ancestor of all, is “biota”, which refers to all life on Earth.
Since this image is so huge, I’ve broken it down into 3 parts:
As we go back through our mammalian history, we can look at these branching points to see where we gained certain traits.
Some examples:
It seems likely that we lost our tails some time before we diverged from gibbons, but after we diverged from Old World monkeys, so 18-30 million years ago.
Tarsiers have multiple pairs of nipples, but only use the top pair, while all simians (monkeys and apes) have only one pair. Some members of strepsirrhini use multiple pairs of nipples to feed their young. So this gives us an idea of how and when we simplified our mammaries down to the familiar pair we have today.
Strepsirrhini (lemurs etc) are the first group to break away from our branch of the primate order, and like most mammals they have wet noses, but haplorhini (tarsiers, monkeys and apes) do not, so it is likely that our noses dried up some time roughly 63 to 66 million years ago. Strepsirrhini are also able to make vitamin C, which haplorhini cannot, which gives us an idea of when we lost that handy ability.
All Boreoeutheria have external testicles, but branches that diverged earlier do not, so we can deduce that the expression “old as balls” equates to around 90-100 million years old.
Placentalia is the clade of mammals that have highly developed wombs, and can carry their infants to full-term without the use of an egg or pouch. The two big groups of placental mammals diverged about 120-130 million years ago.
The Therian subclass refers to all mammals that give birth to live young. This includes marsupials, which do so into a pouch. Before this, monotrema (egg laying mammals, platypuses and echidna) diverged, and it is thought that all mammals laid eggs 200 million years ago. Platypuses and echidna also lack nipples, oozing milk through specialised pores instead, which was the norm for mammals when this split happened.
There are many missing steps at the top of this image, as several branches of proto-mammals went extinct long before we developed boobs, as as noted above we’re ignoring extinct branches. So the next split we see is a big one: Sauropsida represents a huge group of animals, that includes all reptiles, as well as the dinosaurs. Most of the dinosaurs died out 66 million years ago, but a handful survived, and are still around today. We call them birds.
Going back further, and we were semi-aquatic creatures not unlike modern amphibians, whom we diverged from nearly 400 million years ago. We had to lay our eggs in water, as we hadn’t worked out how to have hard eggs with water inside them yet. The group containing mammals, birds, reptiles, and amphibians is call tetrapoda, meaning “four legged”.
And of course, going back further still, we were fully aquatic, as fish. Our closest surviving fishy relatives are the lobe finned fish, which include the lungfish and coelacanths. Our shared ancestor with these fish had lungs, and specially developed fins for life in shallow water and mud. Since we share this ancestor, it can be argued that we, and all tetrapods, are lobe finned fish.
Going back further and we see that we are also in the bony fish family, along with all animals with true bony skeletons. The bony fish are very distant cousins to the cartilaginous fish, which includes sharks, rays, and dogfish.
Keep going back and we find the common ancestors of all animals with a spinal cord. Go back further still and you can see how our lineage split from the many clades of invertebrates that make up the majority of animal life on Earth.
Note: the exact order the bottom 3 groups of this image split off is debated. I picked what seems to be the most widely accepted, but Placozoa in particular are hard to place. They’re very simple animals, being basically tiny clumps of cells, and are not well studied. We only discovered the 2nd, 3rd, and 4th species of them in the last 10 years. Some articles place them as the first to split off from the other animals, while others have them splitting up after sponges but before comb jellies.
The first animals formed as clusters of single celled organisms that started hanging out together. That hang-out gradually turned into a permanent arrangement, which created an animal something like a modern sea sponge. Earlier than this, our ancestors were single celled, which is why most of the images here are unfamiliar blobs, sometimes with tails (flagella) or tiny tentacles (pseudopods). The exceptions are the two other times multicellular life happened: the cells of some fungi also started working together to the point where they became one big organism, which is how we get mushrooms. Going further back we find the branch that leads to algae, some of which grouped up to form plants, the third group of multicellular organisms.
The middle part of this image is where things get really messy, as the classification of Eukaryotes is very much a developing field. I used 3 main sources for this: The New Tree of Eukaryotes Fabien Burki et al.; An excavate root for the eukaryote tree of life by Caesar Al Jewari and Sandra L Baldauf; and Lifemap, an interactive tool that allows you to explore the tree of life. All 3 sources conflict slightly on some points, but I’ve tried to represent the areas they overlap, and in the case of three clades that split off early (Preaxostyla, Fornicata, and Parabasilia), I’ve simply grouped them in one box and admitted that we can’t be sure where to place them yet.
Eukaryotes are the largest and most complex cells, containing membrane-bound organelles like the nucleus, where DNA is kept, and mitochondria, where respiration happens to release energy from glucose. The mitochondria itself has a very interesting history: it used to be a free-living bacteria. It still has its own bacterial genome, entirely distinct from the rest of the cell. The (very much simplified) theory is that long ago, an anaerobic single celled organism (specifically a type of archaea) tried to eat an aerobic species of bacteria, and failed. Instead, the bacteria got stuck inside it, using its aerobic ways to take in oxygen and sugar and release energy for the archaea it lived inside. This was quite the innovation, and this new combo went on to do some amazing things, including form every cell in your body.
This means eukaryotes, often considered one of the 3 domains of life, are actually a sort of symbiotic combination of the two other domains: archaea and bacteria. Here I’ve followed the lineage of that archaea, but added in a little dotted line to show that a bacteria helped out by forming the mitochondria.
And finally we are all the way back at LUCA, the last universal common ancestor of all living things, the grandmother of all grandmothers.
Unless the hypothesis of panspermia is correct, and life on Earth was actually seeded by microorganisms that fell from space, this is the furthest back we can go, and we have covered every single surviving branch of the tree of life that diverged from our own.