Evolution Explained: From Bacteria to Humans
Evolution connects all life on Earth. Whether you're a marine worm or a marmoset, the same genetic code proliferates your DNA.
The Primordial Soup
Once upon a time, at least 3.5 billion years ago, there was a primordial soup. It was a tad flavourless, but it did contain some rather promising ingredients.
The soup became heated by lightning, fire, lava, meteorites, hydrothermal vents, or all of the above. The heat started chemical pathways that drove the formation of molecules called nucleosides.
You may know them as adenine, uracil, cytosine, and guanine. They accumulated in abundance, layer upon layer, as brownish crystals on the surfaces of rocks.
The RNA could create negative blueprints of itself through spontaneous base-pairing; a process that enabled self-replication.
What's more, this single-stranded molecule could loop and fold and make internal bonds between its nucleotides.
These spontaneous reactions saw many RNA machines evolve, possessing different shapes, lengths, and sequences.
Very early on, these multi-talented RNAs provided the means to store protein production codes and catalyse their own reactions. This is theory is known as RNA World.
The First Cell
Besides RNA, the primordial soup also gave rise to long chains of carbon, hydrogen, and oxygen atoms forming fatty acids.
These chains of fat molecules hooked up with phosphate groups to make phospholipids. Crucially, their hydrophilic heads were attracted to water, while their hydrophobic tails were repelled by it.
The soup became a bubble bath. Phospholipids spontaneously formed spherical membranes, and when RNA became trapped inside, the first cells were born.
The earliest evidence of microbial life on comes from a rare 3.48 billion-year-old fossil, containing 11 microbial specimens from five separate taxonomic groups.
"Whoa there!" you exclaim. "Where's all this complexity coming from? We don't even have amino acids... do we?"
We do. It's thought that amino acids were routinely deposited on Earth via meteorites, which were rather massive and frequent during the Late Heavy Bombardment around 4 billion years ago.
Such extra-terrestrial amino acid drops still happen now. In 1969, the Murchison meteorite crashed into rural Australia carrying more than a hundred types of amino acids. NASA's Stardust probe has since found the Wild 2 comet to be spewing out glycine in its ice trail.
With thousands of huge meteorites smacking into the primordial soup, the first cells had all three components they needed to survive: RNA, membranes, and amino acids.
The RNA machines produced long chains of amino acids which folded into proteins. And these proteins took on catalytic and structural roles of their own.
Over time, these early bacteria cells replicated based on the nucleotide sequences in RNA we now call genes. The most successful multiplied in abundance, producing many different species.
It's life, Jim, but not as we know it.
The Three Domains of Life
Millions of years passed, and mistakes in the self-replication of RNA created novel cell features. Some of these cells were so diverse from bacteria, they landed themselves a new classification altogether: archaea.
Bacteria and archaea are both prokaryotes (meaning "before kernel" or "pre-nucleus"). They dominated the tree of life until about 2 billion years ago.
It was a drizzly Tuesday afternoon when an archaeal cell engulfed a bacterial cell. She just clean swallowed him whole, and in doing so produced a whole new domain of life: eukarya.
Both cells found their newfound endosymbiosis rather agreeable. This entirely new mode of survival gave eukaryotes a complex internal structure and a second powerhouse of energy.
You and I are eukaryotes—in possession of cells with symbiotic prokaryotes, a nucleus, and many organelles—just like all protists, plants, fungi, and animals.
How Does Evolution Work?
By now you have an idea of how spontaneous chemical interactions drove the origin of life. So how exactly does evolution come into it?
Let's skip forward in time to visualise evolution in terms of much more familiar organisms—animals.
We distinguish different types of animals based on their physical differences, which arise directly from their genes.
The living environment (including climate, predators, and food supply) defines which physical traits are good and bad for survival. It produces the effects of natural selection.
It's a wild thought. At the most basic level, evolution is fuelled by chaos: by mistakes in genetic replication. It's only natural selection that brings order to evolution.
This was the brilliant insight of Charles Darwin and Alfred Russell Wallace, who concluded that nature prunes away detrimental traits and rewards beneficial ones with survival.
"Survival of the fittest" solves the paradox of how increasingly complex life arises from random mutation.
Let's dive a little deeper into this idea of evolution as a two-step process of mutation and selection.
Step 1. Blind Mutation
When genes are replicated within RNA or DNA, errors creep into the code. Despite in-built proofreading mechanisms, the sheer scale of the business means that mistakes are made.
For instance, if a nucleotide is dropped or added, it can displace the sequence that follows. The gene is translated wrong, cells become short of a critical protein, and disease is born.
But not all mutations are bad. Sometimes, mutations make no difference at all. These are called silent mutations. And sometimes, they lead to the production of new proteins, giving rise to the evolution of new features like scales or teeth.
It doesn't take much. Sometimes, entire chromosomes can be duplicated by accident. And changes in homeobox genes (the "master switches" of physiology) can massively alter body plan. This is how worms gained segmentation and flies gained two sets of wings.
But genetic mutation is always blind, like infinite monkeys smashing at infinite typewriters. Eventually, a monkey types a readable word and we get a new adaptation.
Step 2. Natural Selection
Earth has more than a few different environments: deep sea floors, tropical rainforests, frozen tundra... each with unique flora and fauna exquisitely well-adapted to their habitats.
This is the work of natural selection. When he travelled to the Galapagos Islands in 1835, Darwin saw that different species of finches were uniquely adapted to their different habitats. Fruit-eating birds had short, stubby beaks, while insect-eaters had long, prying beaks. He realised the environment was shaping their physical nature—not individually, but collectively, implying a link between habitat and inheritance.
Environmental pressures provide the self-correcting mechanism for successful mutation. And when sufficient mutations accrue in a population, you get a new species.
Natural selection affects any aspect of survival, from macroscopic traits like brain size, to microscopic traits like blood clotting factors.
Superficially, it can seem like genes are goal-oriented (or selfish in the words of Richard Dawkins). Of course, they're just chemicals in a feedback loop with the environment.
There's no grand plan in evolution, only brutal and continual cycles of trial and error.
The Evolution of Animals
Back to our original story. How did single-celled eukaryotes evolve into the stunning variety of animals we see today?
I'm glad you asked. Because the rest of our journey takes a step-by-step look at the evolution of animals, from the first clumps of cells in the ocean to one of nature's latest whacky ideas: humans.
Life took on new meaning about 635 million years ago, when single-celled eukaryotes called choanoflagellates began living in colonies.
These clusters of cells thrived on the principle of safety in numbers. Eventually, some acquired adaptations that gave them new functions, and work in the colony became specialised. This cell differentiation saw each sub-cluster of cells provide different benefits to the group as a whole.
This is how multicellular animals began. And they're still around today, looking and behaving not very animal-like at all as sponges.
You may not think of sponges as animals. They have no brains, blood, organs, or even true tissues. Yet they are distinctly made of animal cells working together. By Jove, life found a way.
Roald Dahl had it right when he named Aunt Sponge. This is your most distant animal relative in the world.
The Evolution of Radial Symmetry
As sponges mutated and gained complexity and size, some of them took on new qualities like softer bodies and radial symmetry. This symmetry gave them a distinct top and bottom.
A new phylum of animals emerged, called Cnidaria. They include all jellyfish, hydras, anemones, and corals around today.
Cnidarians also evolved specialised stinging cells, called cnidocytes, which help them capture food actively. Stingers marked the dawn of predatory behaviour in animals, a factor that would drive many later species into an evolutionary arms race.
Also for the first time on Earth, animals boasted distinct tissue layers, giving them an internal body cavity for digesting food and transporting nutrients. Now that's some fancy biological equipment.
The Evolution of Bilateral Symmetry
Over millions of years, Cnidarians diversified further into exotic forms. It's thought that hydras evolved into flatworms, which rule their own entire phylum called Platyhelminthes. The major new evolutionary feature of flatworms was bilateral symmetry.
Bilateral symmetry gives an animal mirrored left and right sides. You and I have bilateral symmetry. It's a massively useful adaption that led to the development of a head-end, also known as cephalisation.
Until this point, radial symmetry suited a lifestyle of drifting through the ocean, meeting the environment equally from all sides. But bilateral symmetry and cephalisation lent animals to active movement. They encountered their environment head-on, where all the sensory equipment was located.
With eyes and mouths at the head-end of the flatworm, nerve bundles also became more usefully concentrated in the head too. These clusters of nerve cells produced an early—if primitive—brain.
The Evolution of Parasites
Parasites go way back. It's hypothesised that Nematodes evolved from animals whose development was arrested in the larval stage. However, their true age and precise lineage is unknown because their fossilised remains are just so damn small.
Nematodes are tiny critters, known for their cylindrical bodies and adaptability to a diverse range of habitats. Different species are content living deep in the Earth's crust, in the ocean, or in your gut.
One evolutionary superpower of Nematodes is a tough outer coating called a cuticle which protects their inner organs.
They also hit on the parasitic lifestyle, stealing energy directly from their hosts. Roundworms are simple yet highly adaptive creatures, and that's precisely what makes them so successful.
The Cambrian Explosion
Half a billion years ago, there was a biological Big Bang. The evolutionary tree of life spawned myriad new branches, including dozens of new animal phyla. All the major body plans we see in animals today emerged during this incredible period of evolution. So what kicked it all off?
The Cambrian Period started around 540 million years ago and lasted for about 55 million years. The idea of it being a Big Bang or an explosion is somewhat of a relative metaphor, but in terms of evolution, a lot happened over a very short time.
At the start of the Cambrian era, Earth was cold. But soon, global temperatures climbed, glaciers melted, and sea levels rose.
The shrinking ice sheets allowed more light to penetrate the oceans, having major consequences on aquatic life:
- More sunlight fuelled the growth of more aquatic plants
- More plants provided more grazing food
- More plants created more oxygen via photosynthesis
- More oxygen allowed animals to grow larger
The Cambrian Period was a perfect storm of abiotic (non-living) factors like climate and topography, interacting with biotic (living) factors like the food web and predation. Such feedback cycles drove the rapid diversification of the animal kingdom.
As you'll see in the next few phyla, the rapid co-evolution of animals pushed predators and prey into an arms race. Predators grew bigger, stronger, and faster, while prey developed camouflage, protective shells, and faster reflexes.
Genetic Evolution During The Cambrian Explosion
At the genetic level, things were changing fast. In particular, there were some key mutations in homeobox genes. These control the development of the body plan during the embryonic phase. In other words, they organise the head from the arse.
Because homeobox genes are the master switches of physiology, just small mutations in them can create massive differences to the overall body structure. Homeobox mutations eventually gave bodily segmentation to worms, and an extra set of wings to insects.
Homeobox changes may have been in the making before the Cambrian Period began. When the Earth's environments and ecosystems changed, homeobox variations provided superb fodder on which natural selection could act.
The Evolution of Shells
Sometimes when you walk along the beach you find a really nice shell. These are not geological fancies but rather the calcium carbonate shells of dead Molluscs. That's right—beaches are mass snail graveyards.
The evolutionary features of Molluscs include soft bodies which excrete a hard protective shell. Most species are aquatic (like clams, oysters, and squid) although some live exclusively on land (like your standard garden snail).
Wait a minute, Columbo, did you just say squid?
Yep, squid are Molluscs too. The squid has a small ancestral shell, called a gladius, which became internalised in the course of its evolution. The gladius supports the mantle and serves as a point for muscle attachment.
Squid are incredible creatures that independently evolved eyes. And they can grow huge. In the case of colossal squid, up to 14 metres in length. Squid are closely related to octopuses, who not only have three hearts and blue blood, but have also demonstrated problem solving and tool use. Molluscs can be extraordinarily intelligent creatures.
The Evolution of Body Segments
The early Cambrian Period gave rise to a lot of worms. Annelids, including marine worms and deliciously slimy earthworms, evolved distinct body segments, each one internally and externally identical to the next.
Some marine annelids evolved tiny paddle-like feet for swimming, which would later enable them to push through soil on the land. In time, the evolution of Annelids would create an enormous range of body sizes, from 0.5-millimetre aquatic worms to 3-metre Giant Australian Earthworms.
The Evolution of Appendages
The paddle-like feet of marine worms kicked off an incredible phylum, members of which reside in your home today: Arthropods.
The three main lineages of arthropods are spiders, insects, and crustaceans. Together they account for 80% of all animals on Earth today. In other words, Arthropods rule.
Arthropods first appeared when some Annelids evolved into tiny crustaceans called ostracods, or seed shrimp. Homeobox mutations saw their paddles develop into jointed appendages which later specialised to function as antennae, pincers, mouth-parts, and legs.
Some Arthropods prospered in the water: shrimp, trilobites, and sea spiders still flourish there today. Crabs adapted to spend part of their days on land. Spiders, scorpions, and beetles went terrestrial full-time. And land-dwelling species gave rise to flying insects like bees, butterflies, and mosquitoes.
Arthropods make up a huge and diverse phylum. Yet they all share key features which allow us to trace their common descent. They're exceptional at mastering life on Earth, being the only phylum to truly conquer land, sea, and air.
The Evolution of Embryonic Development
All the animal phyla we've looked at so far go through an embryonic stage known as protostome development. It characterises the way embryonic cells are organised to create a basic body plan with various internal layers.
If we took a spider embryo when it's just eight-cells small, we'd see the cells arranging themselves in a spiral shape. These cells multiply many times to form a hollow sphere, and then fold inwards to create the beginning of the animal's mouth.
The Cambrian boomers changed all that, ushering in a new era of embryonic growth called deuterostome development 500-600 million years ago. You and I are deuterostomes. Our eight-cell stage is defined by a radial pattern, and later, the first infolding is set to become an anus.
Why does this matter? Because it changes the way our internal layers form. Deuterostome development affords us more a complex nervous system and, ultimately, a backbone. This is what separates most vertebrates from invertebrates. And it's all thanks to mutations in the homeobox genes.
Echinoderms—also known as starfish and sea urchins—undergo deuterostome development.
You read that right. Sea stars are pretty advanced animals in the grand scheme of things. They're not vertebrates, but they do mark the evolution of key transitional states on the way towards it.
Besides their evolution as deuterostomes, Echinoderms boast an internal water canal system and tube feet which together enable movement and feeding. They also reproduce sexually by releasing sperm into the ocean, as well as asexually by breaking off limbs and regenerating. Amazing.
The Evolution of Chordates
With a more complex mode of embryonic development at hand, a new phylum called Chordates evolved. It includes a few more bizarre animals, plus many familiar ones, including dinosaurs, ducks, and dingoes.
Chordates possess four key features:
- The hollow nerve chord develops into the brain and spinal chord.
- The flexible notochord develops into the backbone.
- Pharyngeal slits function as gills or an inner ear.
- The post-anal tail aids swimming and balance.
At a glance, early chordates don't look that special. Take a look at the sea squirt or Urochordate. The larval form has all the Chordate features as a tadpole-like creature in search of a landing pad. This immature stage may only last a few minutes. Then something freakish happens.
The sea squirt undergoes a radical metamorphosis, re-absorbing its tail, notochord, and primitive brain back into its body. The rest of its days are spent as an immobile little squirt, siphoning water to filter food, and looking like a gross internal organ while it does so.
Another early Chordate example is the Lancelet, or Cephalochordate. This is a blade-like critter that burrows backwards into the sand, leaving its mouthparts exposed to catch drifting food particles.
Lancelets have no true vertebrae, their brains are small and primitive, and they have pretty dull senses. Yet they do have all the features of Chordates like you and me, reminding us that evolution is a gradual process of cumulative adaptations.
Having glimpsed some of the more alien-like animals throughout our evolution, we now move on to the more familiar. But before we delve into vertebrates, just look how far we've come:
The Evolution of Vertebrates
Take a look at this beauty queen.
The hagfish, or Myxini to zoologists, is basically a swimming sausage. He's a jawless fellow with a partial skull made of cartilage and the ability to produce defensive slime.
What makes him special is the presence of rudimentary pseudo-vertebrae. Unusually for a sausage, he also has a brain, eyes, and other sensory organs. He's elusive in the fossil record, yet alive (extant) today, providing physiological evidence for his evolutionary origins.
Our next guest is also a face for radio.
Lampreys, or Hyperoartia, are early adopters of true backbones. The fossil record reveals their shape has stayed virtually unchanged over hundreds of millions of years of evolution.
Humans descended from a sister animal of lampreys, who was the first to develop an adaptive immune system, allowing our bodies to recognise and remember pathogens today. Although now extinct, they likely looked very similar to lampreys, meaning this is what your great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-grandmother looked like. You really have her eyes / tail / sucker mouth.
The Evolution of Jaws
About 430 million years so, natural selection favoured the evolution of jaws and a mineralised skeleton. Welcome to the age of true predators.
Early Chondrichthyes, like sharks and rays, were able to eat big chunks of flesh, so grew bigger and swam faster compared to their ancestors. With such exotic and fast-moving predators emerging, evolution raced along (relatively speaking, of course).
The Evolution of Lungs
You may have noticed that we're still largely in the water along our evolutionary trail. That's because aquatic animals had not developed any capacity to gulp air. Until now.
Sarcopterygii are muscular, lobe-finned fish with bony skeletons, such as lungfish and coelacanths. The latter was thought to have been extinct for 65 million years until, in 1938, a very confused fisherman hauled one in and took it to a local naturalist for identification.
All other fish fall into the phylum of Actinopterygii, or ray-finned fish, which evolved manoeuvrable fins and a swim bladder. For many fish, the swim bladder is a buoyancy aid; an air-filled sac which keeps them at their water depth without having to waste energy on swimming. However, for some, it performs as a rudimentary lung.
The Evolution of Tetrapods
By now, the most complex life forms on Earth had eyes, brains, backbones, jaws, gills, lungs, and fins. Only relatively small modifications of this body plan were needed to produce the superclass known as Tetrapods (meaning "four feet").
The recent fossil discovery of Tiktaalik provided the so-called missing link between aquatic lobe-fins and land-dwelling Tetrapods. Like a fish, the extinct Tiktaalik had fins, gills, and lungs, and its body was covered in scales. But unlike a fish, it had rudimentary ribs to ventilate its lungs, seriously muscular fins to support its body out of the water, and a neck and shoulders to help move its head.
Notably, Tiktaalik's fin-feet already had the bone structure common to Tetrapod wrists today. The fellow gave rise to the first Tetrapod land dwellers known as Amphibians.
Amphibians include frogs, salamanders, and caecilians. While reasonably comfortable on land, their lives are inextricably connected to the water. Their moist skin and eggs are vulnerable to drying out, so they can never truly explore the far reaches of dry land.
Amphibians also go through quite the metamorphosis early in life, having aquatic larval forms which are distinctly different from their adult forms. Such convoluted lifecycles are evolutionary throwbacks to older animals like jellyfish and tunicates.
The Evolution of Amniotic Eggs
If moist eggs tied Amphibians to water, then hard-shelled eggs would liberate their Reptiles descendants.
Reptiles include some of the most objectively fun animals that ever existed: dinosaurs, snakes, lizards, crocodiles, and turtles.
Consider Testudines for a moment: the bizarre shelled-reptiles known as turtles, tortoises, and terrapins. Once possessing teeth, Testudines lived alongside the dinosaurs and survived the mass extinction event that killed its Reptile cousins.
Dinosaurs are a completely different evolutionary branch of Reptiles that lived during the Mesozoic Era. Dinosaurs are split into two major groups:
- Ornithischians (meaning "bird-hipped") were mostly herbivores. Some Ornithischians, like Triceratops and Ankylosaurs, evolved thick skulls and armoured plates which protected them from predation.
- Saurischians (meaning "lizard-hipped") included carnivores like Tyrannosaurus Rex and Giganotosaurus, as well as massive long-necked herbivores like Diplodocus and Brachiosaurus.
The Evolution of Flight in Vertebrates
A number of dinosaurs are known to have evolved wings. For instance, Pterosaurs evolved flight some 230 million years ago, while Microraptors had feathered wings for gliding about 120 million years ago.
So how and when did birds evolve from dinosaurs?
As Alan Grant taught us in Jurassic Park, Aves descended from dinosaurs. In fact, Archaeopteryx is widely considered the first bird, and as you very well know, Archaeopteryx was also a dinosaur. As a result, most palaeontologists now accept that birds are a specialised group of dinosaurs that arose 160 million years ago.
Modern birds evolved from as few as three dinosaur lineages about 95 million years ago during the end of the Cretaceous Period. They have some amazing adaptations to aid flight, including hollow bones, loss of teeth, a single ovary in females, and aerodynamic feathers.
Flight has massive survival benefits in terms of hunting, escaping predation, and migration. Yet some predator-free environments have caused birds to become flightless. When it comes to evolutionary adaptations, you have to use it or lose it.
Kiwis are one of many flightless bird species native to New Zealand, which evolved in geographical isolation for 80 million years. When the Polynesians arrived on the islands with rats in the 13th century, native bird populations took a big hit. The introduction of cats and dogs by European settlers in the 18th century only exacerbated the problem.
Extinctions happen for a number of reasons, but there's always one common feature: the environment changes too rapidly for organisms to adapt. Humans certainly aren't making it easier.
The Evolution of Mammary Glands
By definition, Mammals are animals which have hair and produce milk from mammary glands. I don't know my dad's excuse.
Pseudo-mammals evolved from Reptiles 178 million years ago. By the end of the Triassic period, Mammals were small, hairy creatures which fed on insects at night and still laid eggs. In time, they diversified to a degree, but competition and predation was fierce: dinosaurs already dominated many ecological niches.
Three lineages of Mammals—Monotremes (egg-layers), Marsupials (pouch-bearers), and Eutherians (placenta-bearers)—were established when most dinosaurs went extinct due to sudden environmental shifts, probably involving a very big meteor.
The Mammals that survived exploited the ex-dinosaur habitats, food sources, and territories, rapidly filling the ecological niches left behind. This was our chance to shine.
Over the next 65 million years, Mammals diversified into a range of forms and lifestyles. Rodents took to living underground. Primates took to the trees. And Cetaceans said "sod it" and went back to the oceans to become dolphins and whales.
It wasn't until 4.4 million years ago when Hominins emerged in the form of Australopithecus. These unusually smart African apes were likely the direct ancestors of modern humans. They thrived for 3 million years before disappearing at the hands of global climate cooling, or competition with other species of Hominins, or both.
Homo sapiens emerged just 200,000 years ago, according to fossil remains found in Africa. Yet it hasn't been a smooth ride for us either.
Around 75,000 years ago, our numbers were catastrophically diminished to between 3,000 and 10,000 individuals at the hands of the Toba supervolcano eruption in Indonesia. We hit a genetic bottleneck, evidence of which exists in your DNA today.
Natural climate change caused by the Toba eruption likely forced the surviving humans to adopt new survival strategies. These behavioural adaptations may even have driven us to replace the Neanderthals and other archaic human species.
Congratulations for getting this far. You, sir, are fit to survive. Every single ancestor of yours has matured and reproduced without fail for at least 3.5 billion years straight.
They didn't know it, but they were all working towards you. (And the guy at the petrol station. And your barber. And the old lady who stalks you on Facebook. Ok, it doesn't sound so special anymore. But if you can have an ego about it, it can all seem rather wonderful.)
The fact that we exist means we've won the evolutionary lottery. All 7.8 billion of us. Even if we do all hark back to a slimy old hagfish.
Are Humans Still Evolving?
The zoologist David Attenborough suggests that, for the first time, humans may have stopped evolving.
Wait a second—what?
We still make mistakes in our DNA replication. And this still creates diseases and adaptations. And let's not forget that evolution takes a heck of a long time to ripple through populations.
But take a snapshot of this moment in time and we can observe our species wielding considerable control over our environment. The effect of natural selection is, for now, significantly diminished.
For billions of humans, agriculture and shipping ensure a steady food supply. Vaccines provide herd immunity against infectious disease. Criminal justice helps maintain social order. And gene editing allows us to correct disease mutations, locking out major drivers of natural selection.
As long as our society and technology hold strong, Mother Nature can no longer kick us out of the gene pool. Instead, we swim to our hearts' content, few of us pausing to realise the sheer multitude of organisms that have lived and died before us at the mercy of evolution.