Click on all links: A look at the latest version of the Tree of Life
Even hugely incomplete, it is teaching us… how little we know, how differently live has behaved. It may even hold clues to how humans might evolve next.
It started quietly; with single cells dividing in the dark.
From that microscopic revolution came multicellular life; the strange Cambrian creatures that once ruled the seas; the fish that crept onto land; early reptiles; furry mammals that scurried beneath dinosaurs; apes, elephants, whales, giraffes, humans.
We may never have a complete map of exactly what goes where, on the grand, sprawling expanse that is this “tree of life”.
There have been times when we weren’t even sure it was a single tree. But data shows it is. And now, we’re parsing information in milliseconds, building branches armed with more information on a single species than entire libraries contained just two centuries ago.
Max Telford represents the most exciting phases in this journey, in his debut book, The Tree of Life: Solving Science’s Greatest Puzzle.
The British evolutionary biologist teaches zoology and comparative anatomy at University College London, and has spent three decades studying the tangled bonds that link living things. In his book, he zooms out: what exactly have we learnt about how all living beings — bacteria, plants, worms, mushrooms, humans; past and present — are connected?
This puzzle has occupied scientists since Charles Darwin first sketched a spindly tree in his notebook in 1837, alongside the note: “I think.”
LEAF OF FAITH
Darwin’s doodle has been replaced by vast computer-generated diagrams.
The latest is a swirling shape reminiscent of a fern.
On this expansive tree of life, all forms fall into three groups: bacteria, archaea (single-cell organisms that lack a nucleus, but are genetically and chemically different from bacteria), and eukaryotes (animals, plants, fungi and everything else with complex cell structures).
By number, the tree is overwhelmingly microbial. Bacteria account for the bulk of species. Animals, by comparison, make up only the thinnest sprays of leaves at the tips of a few branches. Humans are one twig on one branch of one minor bough, within the category eukaryotes.
All this constitutes a fraction of 1% of the estimated 1 trillion species alive today.
Meanwhile, it has taken nearly 200 years, and considerable trial and error, to make it even this far.
Before gene mapping, as Telford documents, scientists attempted to build the tree by comparing morphology: legs, wings, shells, flowers, bone structures.
With molecular biology, researchers could finally compare organisms’ genetic make-up, mapping proteins, RNA, DNA, and eventually genomes (all the genes that make up a specific species). This has been very recent.
“When I started my PhD in 1989, we were just beginning to use genes to work out how animals are related,” Telford says. “My doctoral thesis focused on a molecular analysis of the arrow worm, using just one gene, where now we might use a thousand. Very soon, we will be using entire genomes in such studies.”
PROBABLE CAUSE
Even with all the additional data available, and the artificial intelligence to help analyse it, the puzzle remains mind-boggling.
“If you take just three species — a human, a chimpanzee, and a gorilla — there are three possible ways they could be related,” Telford says, trying to explain the scale of the problem. “Add an orangutan and there are suddenly 15. Add one more species and there are 105. For the 28 known species of apes, the number of possible trees is something like 1, with 35 zeros after it.”
This hasn’t stopped biologists from trying. And already, as branches take shape, surprises are tumbling from them.
It turns out, for instance, that the grey wolf native to Eurasia and North America is more closely related to the whale than to the thylacine or Tasmanian tiger — a reminder that nature has yielded the same form, from different raw materials, over and over (in what is called convergent evolution).
Meanwhile, swifts, so similar to swallows that even the father of modern taxonomy Carl Linnaeus grouped them together, are actually closer in genetic makeup to hummingbirds, while swallows are distant cousins of the owl. Once again, the same form emerges from species that are nothing alike, in different parts of the world.
THE WORKINGS OF THE WEB
What else could a tree of life teach us?
Well, it’s already proving that moving forward isn’t always the best option.
We know evolution isn’t about advancement, but about survival. So, if shedding a few characteristics and simplifying form helps, that’s what happens.
Just how far “backwards” can such a creep go?
“A lot of my work has been showing that things placed at the bottom of the tree — because they look simple — were actually once complex animals that threw away a lot of their complexity,” Telford says.
This has happened, over and over, to such a dramatic degree that the mint-sauce worm, for instance, long believed to be a primitive leftover from the dawn of life, is now thought to be descended from far more complex ancestors, with possible genetic links to the starfish, and perhaps even to certain vertebrates.
Why would it have taken so many steps backwards? The answer to that could tell us a lot about how life could change, as the planet does.
There is hope that the tree can also answer questions about the origins of evolutionary oddities, such as the human chin. Why do we have it? “No one really knows,” Telford says. “There are theories that it might help with speech, or might have helped us withstand attacks better. Perhaps it was an accident,” he adds, laughing. “Compare a human face and a chimp face, and their jaw and lower face stick out much more than ours. Maybe as our face shrank, the chin didn’t get the message.”
DEAD ENDS
The really big challenge is filling in the earliest years.
Most early forms consisted of soft-bodied organisms that left no fossils, erasing entire branches from our view. Yet, while the past remains murky, the tree could help us figure out what we may become next.
“Maybe humans will split into two species,” Telford says. Maybe we will simply shape-shift again. “Or maybe we won’t be around…”
What is certain, he notes, is that one day, the entire tree will be erased.
If this sounds overwhelming, that’s partly the point. We should be fascinated by this “most extraordinary adventure of life”, but also a little humbled, Telford says.
“We’re obviously very important to ourselves,” he adds. “But the truth is that we are one, single, very new piece in this enormous puzzle. That really gives one perspective on humanity’s place in the world. We might not be very important after all.”
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