Evolution Explained: From Bacteria to Humans

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.

Once upon a time, at least 3.5 billion years ago, there was a primordial soup. This soup contained all your favourite base ingredients.

Ingredients in The Primordial Soup

The soup became heated. We don't know the source, but there are plenty of contenders: lightning, fire, lava, or hydrothermal vents would have done the trick. The heat initiated chemical pathways that prompted the spontaneous formation of nucleotides from carbon, hydrogen, nitrogen, and oxygen.

These nucleotides are still present today. You may know them as Adenine (A), Uracil (U), Cytosine (C), and Guanine (G). They accumulated in abundance, layer upon layer, as brownish crystals on the surfaces of rocks.

Eventually the nucleotides reacted with phosphorus (from the rocks) and ribose rings (from the soup) to form the first RNA string.

The First RNA and The Origin of Life

Much of this process has been repeated in the lab in the 21st century. It wasn't a miracle, so much as a chemical inevitability...

In time, the churning soup produced more variations of RNA with varying shape, length, and sequence. Over millions of years, an array of RNA machines emerged. This is theory is known as RNA World.

The First Cell

Meanwhile, long chains of carbon, hydrogen, and oxygen atoms formed in the soup to make fatty acids. They joined with phosphate groups to make this little guy, who would be essential in making the first cell.

Phospholipid Diagram

You've seen it happen in your bubble bath: lipid molecules form spheres when they come into contact with water. The same hydrophobic interactions caused phospholipids to form tiny bubbles in the primordial soup, giving rise to the first cell membranes.

When molecules of RNA became trapped inside these membranes, the first cells were born.

The First Cell on Earth

They were basic cells, but cells nonetheless. Bacteria, in fact, complete with long strands of RNA. Crucially, the nucleotide sequence of the RNA attracted a specific set of complementary amino acids. Driven by chemical attractions, these amino acid chains folded into proteins; the building blocks of organisms.

This is called abiogenesis: the origin of life from non-living molecules.

The race to evolve began.

The Three Domains of Life

Over hundreds of millions of years, mistakes in RNA replication gave rise to more novel cell features. Some of these cell species were so diverse from the original bacteria, they landed themselves a new name altogether: archaea.

It was a drizzly Tuesday afternoon when an archaeal cell engulfed a bacterial cell. She just clean swallowed him whole inside her membrane.

Endosymbiotic Theory - The First Eukaryote

It was a total accident. Yet both cells found their newfound endosymbiosis rather agreeable. The archaeon derived more energy from her new tenant, while the bacterium was protected inside his new house.

It was an entirely new mode of survival, giving the new eukaryotic cell a more complex internal structure and a second powerhouse of energy.

Early eukaryotic cells were probably amoeba-like organisms. In time, they clustered into colonies, creating the first multicellular eukaryotes.

Today, bacteria, archaea, and eukarya make up the three domains of life. You, sir, are a eukaryote.

The Domains of Life - Bacteria, Archaea, Eukarya

How Does Evolution Work?

By now you have an idea of how spontaneous chemical interactions drove the origin of life. So where does evolution come into it?

Evolution is the gradual change in how species look and behave over time. It's caused by genetic mutation and refined by the environment.

How Does Evolution Work?
It's a wild thought. At the most basic level, evolution is fuelled by chaos: by mistakes in genetic replication.

Natural selection brings order to that chaos. The theory of evolution was around for a century before Darwin and Wallace independently identified how nature prunes away detrimental traits and rewards beneficial adaptations.

Darwin and Wallace Cartoon

Evolution is fundamentally a two-step process:

1. Blind Mutation

Some mistakes in DNA replication cause problems like disease. Even the erroneous insertion or deletion of a single nucleotide can muddle the sequence that follows, throwing long sequences (genes) out of whack. Examples of single gene mutations in humans include cystic fibrosis and colour blindness.

But not all mutations are bad. Just look at the X-Men. Some nucleotide changes make no difference; these are silent mutations. Others produce new proteins, or shift the organisation of body plan, translating to adaptations like an extra toe or faster reaction time.

Mutation is always blind, like infinite monkeys smashing at infinite typewriters. Eventually, a monkey will type a readable word. It wasn't deliberate, and yet it produces a meaningful combination of letters.
Infinite Monkeys on Typewriters

2. Natural Selection

Organisms with disease mutations are less likely to thrive, and their genes are sooner flushed out of the gene pool. Conversely, adaptations make them better suited to their environment, raising their chances of survival and reproduction.

A well-camouflaged iguana has better odds of evading the eye of a predator. He's more likely to reproduce and pass on his mutant genes. Nature has selected him for success.

Together, blind mutation creates the diversity of life, and natural selection provides the self-correcting mechanism for adaptive progress. When sufficient meaningful adaptations accrue in a population, they become a new species.

So while it may seem like genes are goal-oriented (or "selfish" according to Richard Dawkins), they're really just chemicals in a feedback loop. There's no grand plan in evolution; only changing life forms shaped by their changing environment.

The Evolution of Animals

Back to our story. How did the single-celled eukaryotes evolve into the variety of plants, fungi, and animals on Earth today?

I'm glad you asked. While some eukaryotic cells thrived in the singular form, others adapted to live in colonies, producing the first multicellular organisms.

  • Plants started out as algae in the oceans, before being spit out onto the land to evolve as mosses. The evolution of roots took them to new heights: first as vascular plants like ferns, then seed plants like conifers. Flowering plants came just 250 million years ago—some dinosaurs never saw flowers.
  • Fungi likely began as aquatic cells with tails, affording them motility. They, too, went colonial and took to the land, evolving into complex yeasts, moulds, and mushrooms which play critical roles in our ecosystem today.
  • Animals emerged about 1.2 billion years ago, when single-celled eukaryotes called choanoflagellates landed on the smashing idea of living in colonies...

These clumps of cells eventually led to the most basic group of animals: Porifera, or sponges to you and me.

You may not think of sponges as animals. They have no brains, blood, organs, or even true tissues. Yet they did evolve specialised cells within their colonies, performing complementary functions between them.

Roald Dahl had it right when he named Aunt Sponge. This is your most distant animal relative in the world.

Porifera Evolution Explained

The Evolution of Radial Symmetry

As sponges mutated and branched into many different variations, the mutants among them took on new qualities like softer bodies and radial symmetry. This symmetry gave them a distinct top and bottom. Like a pizza, they could be sliced up equally along many axes and still look the same.

Some of these mutants blindly evolved stinging cells, or cnidocytes, which helped them capture food actively, instead of filtering particles passively from the ocean.

It was a remarkable adaptation for survival, and a new group of animals emerged: Cnidaria. This next major group of animals, known as a phylum includes all jellyfish, hydras, anemones, and corals. These are very ancient animals indeed.

Cnidaria Evolution Explained

For the first time on Earth, animals now boasted distinct tissue layers. It gave them an internal body cavity for digesting food and transporting nutrients.

Now that's some fancy biological equipment.

The Evolution of Parasites

Parasites go way back. It's theorised that Nematodes evolved from other animals which stopped developing 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.

Nematode Evolution Explained

One evolutionary superpower of Nematodes is a tough outer coating called a cuticle which protects their inner organs. They were also the first animals to hit on the parasitic lifestyle, drawing resources straight from their hosts. Roundworms are simple yet highly adaptive creatures, and that's precisely what makes them so successful.

The Evolution of Bilateral Symmetry

Evolution requires long stretches of time. Just imagine how long it took for those rambunctious monkeys to accidentally type new actual words. And then those words—those adaptations—had to propagate through entire populations before a meaningful number of individuals possessed the mutant traits.

Fortunately, time was on our side. Over millions of years, cnidarians diversified further into exotic forms. Hydras evolved into flatworms, or Platyhelminthes, when genetic shuffling produced a new feature called bilateral symmetry.

Platyhelminthes Evolution Explained

Bilateral symmetry gives an animal mirrored left and right sides. You and I have bilateral symmetry. It's a massively useful adaption that soon led to the development of a head-end. This is 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 soon became 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, some 560 million years ago.

The Cambrian Explosion

Half a billion years ago, there was a biological Big Bang where the evolutionary tree of life spawned myriad new branches. 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 the Earth's lifespan, a lot happened over a very short time.

The Cambrian Explosion Graph

At the start of the Cambrian era, Earth was cold. But soon, global temperatures climbed, glaciers melted, and sea levels rose.

The decrease in ice sheets allowed more light to penetrate the oceans, fuelling the growth of more aquatic plants. This provided more food for aquatic grazers, while increasing oxygen levels in the ocean via photosynthesis. As an essential fuel for animal metabolism, more oxygen meant animals grew larger and pursued more energy-intensive lifestyles.

Such feedback cycles may have driven the rapid diversification of the animal kingdom. And so 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.

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.

At the genetic level, things were changing fast. Of particular note are changes to developmental genes called 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 they're master switches of physiology, just small mutations in homeobox genes can create massive differences in overall body structure. Mutated homeobox genes eventually gave segmentation to worms, and an extra set of wings to insects.

Such mutations may have been in the making before the Cambrian Period began. But 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 Molluscs.

Despite first appearances, Molluscs are an interesting and diverse phylum. Their special features include soft bodies which excrete a hard protective shell. Most Molluscs are aquatic (like clams, oysters, and squid) although some live exclusively on land (like your standard garden snail).

Mollusc Evolution Explained

Wait a minute, Columbo, did you say squid?

Yep, squids are molluscs too. They have a small ancestral shell called a gladius which was internalised in the course of their evolution. The gladius supports the squid's mantle and serves as a point for muscle attachment.

Squids are incredible creatures who independently evolved eyes and, in the case of colossal squids, grow up to 14 metres in length. They're closely related to octopuses, who not only have three hearts and blue blood, but have also demonstrated problem solving and tool use.

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.

Annelid Evolution Explained

Some marine annelids evolved tiny paddle-like feet for swimming, which would later enable them to push through soil on the land. Further mutations would create an enormous range of body sizes, from 0.5-millimetre aquatic worms to the 3-metre Giant Australian Earthworm.

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, and together they account for 80% of all animals on Earth today. In other words, Arthropods rule.

Arthropods first appeared when some Annelids evolved into small crustaceans. Homeobox mutations saw their paddles develop into jointed appendages which gradually specialised to function as antennae, pincers, mouth-parts, or legs.

Arthropod Evolution Explained

Some Arthropods prospered in the water: shrimp, trilobites, and sea spiders still hang out there today. Some, like crabs, adapted to spend part of their days on land. Others, like spiders, scorpions, and beetles, went terrestrial full-time. Later, 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 also 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 embryonic stages of protostome development. It characterises the way 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.

Protostome Development Illustration

Our next Cambrian boomer changed all that, ushering in a new era of embryonic growth called deuterostome development. 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.

Deuterostome Development Illustration

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 vertebrates from invertebrates. And it's all thanks to mutations in homeobox genes.

The first animal to evolve deuterostome development was the humble Echinoderm—also known as starfish and sea urchins.

Echinoderm Evolution Explained

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 to it.

Besides their breakthrough 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

Once starfish set the world of embryonic development on fire, a new phylum called Chordates started to evolve. It includes a few more bizarre animals, plus many familiar ones, including dinosaurs, ducks, and dingoes.

Chordates possess four key features:

  • A nerve chord which develops into the brain and spinal chord.
  • A flexible notochord which develops into the backbone.
  • Pharyngeal slits which function as gills or an inner ear depending on your species.
  • A post-anal tail initially for swimming, though a quick feel of your rear will confirm it's reduced in humans.

At a glance, early chordates don't look that special. Cephalochordates like lancelets are blade-like critters who burrow backwards into the sand, leaving their mouthparts exposed to catch food particles.

Cephalochordate Evolution Explained

They 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.

Another Chordate example is the sea squirt or Urochordate. The larval form has all the Chordate features, swimming around 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 eat, and looking like a gross internal organ while it does so.

Urochordate Evolution Explained

Having glimpsed some of the more alien-like animals throughout natural history, we now move on to the more familiar. But before we delve into the evolution of vertebrates, just look how far we've come...

Invertebrate Evolution

The Evolution of Vertebrates

Take a look at this beauty queen.

Myxini Evolution Explained

The hagfish, or Myxini to zoologists, is a jawless fellow with a partial skull made of actual cartilage. It's an aquatic, slime-producing animal that, while hundreds of millions of years old, still exists as a number of species today.

The standard hagfish has rudimentary vertebrae, a brain, eyes, and other sensory organs. In evolutionary terms, he's kind of a big deal.

Our next guest is also a face for radio.

Hyperoartia Evolution Explained

Lampreys, or Hyperoartia, are other early adopters of backbones. This is your great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-grandmother. 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 was racing along.

Chondrichthyes Evolution Explained

The Evolution of Lungs

You may have noticed that we're still largely in the ocean along our evolutionary trail. That's because sea animals had not developed any capacity to gulp air. Until now.

Sarcopterygii are muscular, lobe-finned fish, the most famous of which is the coelacanth, thought to have gone extinct along with the dinosaurs 65 million years ago. In fact, they were discovered to be alive and well when, in 1938, a fisherman hauled one in and took it to local naturalist.

Sarcopterygii Evolution Explained

Soon, the ray-finned fish, Actinopterygii, evolved the unique traits of 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. It is also a rudimentary lung.

Actinopterygii Evolution Explained

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.

The recent fossil discovery of Tiktaalik provided the so-called missing link between aquatic lobe-fins and land-dwelling Tetrapods. Like a fish, 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: Amphibians.

Amphibian Evolution Explained

Amphibians includes all frogs, salamanders, and caecilians (legless snakes). 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 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 descendants: Reptiles.

Reptile Evolution Explained

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. These are seriously ancient lizards: the oldest sea turtle fossil dates back 120 million years. It lived alongside dinosaurs and its body plan remains virtually unchanged today.

Testudines Evolution Explained

Dinosaurs are a completely different evolutionary branch of Reptiles which lived between 230 and 65 million years ago, a time collectively known as the Mesozoic Era. We can split dinosaurs into two major groups.

  • Ornithischians (meaning "bird-hipped") were mostly herbivores named for having a pelvic structure similar to that of birds. Some Ornithischians, like Triceratops and Ankylosaurs, evolved thick skulls and armoured plates which protected them from predation by carnivorous dinosaurs.
  • Saurischians (meaning "lizard-hipped") included beastly carnivores like Tyrannosaurus Rex and Giganotosaurus, as well as massive long-necked herbivores like Diplodocus and Brachiosaurus.

A number of dinosaurs are known to have evolved wings. Pterosaurs went ahead and evolved flight some 230 million years ago, while Microraptors had feathered wings for gliding about 120 million years ago.

So where do birds come into this?

As Alan Grant taught us with his renegade theory, Aves descended from dinosaurs. In fact, Archaeopteryx is widely considered the first bird, and as you very well know, Archaeopteryx was a dinosaur. As a result, most palaeontologists now accept that birds are a specialised group of dinosaurs that arose 150 million years ago.

Modern birds evolved from as few as three dinosaur lineages some 60 million years ago, after 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.

Aves Evolution Explained

Flight has massive survival benefits in terms of hunting, escaping predation, and migration. Yet certain predator-free environments have allowed some birds to become flightless. You have to use it or lose it.

Kiwis are one of many flightless bird species native to New Zealand, who evolved in geographical isolation for 80 million years. When the Polynesians arrived with rats in the 13th century, native bird populations took a hit. The introduction of cats and dogs by European settlers in the 18th century only made things worse.

In nature, 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 go back at least 180 million years. By the end of the Triassic period, mammals were small, hairy creatures which fed on insects at night and likely 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 already established when most dinosaurs went extinct due to major 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.

Mammal Evolution Explained

Over the next 65 million years, Mammals diversified to become some of the most intelligent animals on the planet. 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. Mammals have done pretty well since the dinosaurs, and all it took was a well-placed meteor.

This brings us to the present day. Congratulations for getting this far. You, sir, are fit to survive. Every single ancestor of yours has matured and reproduced without fail for 3.5 billion years.

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...

Vertebrate Evolution

Are Humans Still Evolving?

The zoologist David Attenborough suggests that, for the first brief time since life began, 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 diseases. Criminal justice prevents dangerous individuals from reoffending. And gene editing allows us to correct disease mutations, and even our entire germ lines.

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.

Becky Casale Bio

ABOUT THE AUTHOR: Becky Casale is the creator of Science Me. She's studying for a BSc and raising two small humans so parts of her DNA can live on a bit longer.