A backbone and a bite

Feel your own spine

Reach back and press your fingers into the base of your neck. The row of hard bumps you feel is the top of your spine, the tips of a stack of small bones called vertebrae. Nearly every large animal you have ever seen has the same stack inside it: a goldfish, a frog, a crocodile, a sparrow, a mouse. Animals with a backbone are called vertebrates, and this course is about how we came to be one.

Biologists have described about 77,000 living species of vertebrates. Almost half of them, about 37,800, are fish. Reptiles come next with about 12,000, then birds with about 11,000, amphibians with about 9,000, and mammals, the group we belong to, with only about 6,800. Mammals look like the winners because a few of them, such as mice, cattle, and people, are enormously numerous. But the number of kinds is a different count from the number of animals, and by kinds, mammals are the smallest of the five.

All of these animals are branches of one family tree. The usual way to tell our part of that story is from the top down, “from ape to human.” Paleontologists, the scientists who read ancient life from fossils, tell it from the bottom up, and the ape turns out to be only the last few steps. The title of this course, “From fish to humans,” is not a joke about mermaids (no mermaid is involved). It is the outline of the tree: a fish, then an animal with legs, then one whose egg no longer needs water, then a mammal, then an ape, then us. About 520 million years in all.

How to read a fossil

A fossil is any trace of ancient life preserved in rock: a bone, a shell, a leaf, a footprint, a burrow. Most animals that die are eaten or rot away. A fossil forms when a body is buried quickly and minerals seep through it, slowly, over a very long time. How long is “long”? There is no sharp rule, but many scientists use about 10,000 years as the minimum, and the fossils in this course are millions or hundreds of millions of years old. A chicken bone buried in the garden and dug up a week later is still just a chicken bone. A forger can copy the shape of an ancient bronze vessel, but he cannot copy its age. Time is the one thing that cannot be faked, and it is what makes a fossil.

Fossils record two histories at once. One is the history of life: which animals lived when, and how one kind became another. The other is the history of the planet. Wood that has turned to stone shows that forests once grew where there is now desert; in Arizona’s Petrified Forest National Park, the trees fell more than 200 million years ago. And on the roof of the world, Chinese geologists surveying the Qomolangma (Everest) region in the 1960s found the bones of an ichthyosaur, a Triassic marine reptile, in rock thousands of meters above the sea. A sea animal in the Himalayas means the rock was once seabed. The mountains rose where an ocean used to be.

How can anyone know what a fossil animal was like? By comparison. A paleontologist measures the bones and asks which living animal they most resemble, and how closely. This course does it at every step: first the fossil, then a living cousin that helps explain it, such as a lamprey, a coelacanth, or a platypus.

A day of vertebrates

The earth is about 4.54 billion years old. Life appeared on it at least 3.5 billion years ago, and for most of the time since, life stayed small and mostly single-celled. Fish-like animals with the beginnings of a backbone appear in the fossil record about 518 million years ago. That is a span no one can feel, so squeeze it into one day.

Midnight is the first vertebrate, a fish shorter than your thumb. The first jawed fish appear around 3:35 in the morning. The oldest known footprints of four-legged animals come at about 5:40, and the first animals whose eggs can survive on dry land at about 9:15. Dinosaurs arrive in the early afternoon and the first mammals a little later. Archaeopteryx, an early bird, flies at 5 p.m. The asteroid that ends the dinosaurs falls at about five to nine in the evening. The line that leads to us separates from the chimpanzee line about twenty minutes before midnight, and our own species, Homo sapiens, arrives about 50 seconds before midnight. Everything humans have ever written down happens in the last second.

Two things stand out. The first is that nearly the whole story happens without us: for 99.9 percent of the day there is no human. The second is that the dinosaurs, which seem like a brief prelude, hold the afternoon and evening for about seven and a half hours.

Nine turning points

Paleontologists often pick out turning points, moments when a new body part opened up a new way of life. This course follows nine of them, in four lessons:

1. The backbone (about 518 million years ago): a stiff, flexible rod inside a small swimming body.
2. Jaws (by about 440 million years ago): a hinged mouth that can bite.
3. From water to land (about 395 to 365 million years ago): fins become limbs with fingers and toes.
4. The amniotic egg (about 320 million years ago): an egg that can be laid away from water.
5. Back to the sea (from about 300 million years ago): land animals return to the water and change again.
6. Mammals (over 200 million years ago): fur, milk, and warm blood.
7. Flight (about 230 million years ago in pterosaurs, later in birds and bats).
8. Feathers (before flight): from warmth and display to wings.
9. Humans (the last seven million years): upright, large-brained, and curious.

Lessons 1, 2, 3, and 4 take them in order. Two cautions go with the list. A turning point is not a single day: each one took millions of years and left many half-finished animals behind. And it is seen in hindsight. Nobody was heading toward the next step; each animal was just trying to eat, avoid being eaten, and leave offspring. Lesson 4 returns to this, because it is the most common mistake people make about evolution.

The first backbone

The first turning point has a precise address. In Chengjiang county, Yunnan province, south of the city of Kunming, a hillside of fine mudstone called Maotianshan holds one of the richest fossil beds on earth. In 1984 the paleontologist Hou Xianguang recognized how well preserved the animals in it were, down to legs, eyes, and guts, and the site has produced hundreds of species since. UNESCO made it a World Heritage Site in 2012. The animals lived in a shallow sea about 518 million years ago, in the early Cambrian, when most of the body plans of modern animals had just appeared. A sponge-covered seafloor crawled with trilobites and odd soft-bodied animals that left no descendants. Swimming above it was Anomalocaris, a predator some forty centimeters long with a pair of grasping arms and a circular mouth, one of the largest hunters of its day.

And in the same kind of mud near the village of Ercai, by the town of Haikou on the shore of Lake Dian in the Kunming area, lay the animal this lesson is looking for. Haikouichthys (海口鱼 Hǎikǒuyú, named for that Haikou, not the city on Hainan island) was described in 1999 by Shu Degan and colleagues, along with a close relative called Myllokunmingia. It was about 2.5 centimeters long, the length of a large paper clip. Its body was a row of V-shaped muscle blocks, like a fillet of salmon, with a continuous fin along back and tail, a row of gills, a pair of large eyes, and, down its middle, a stiff but bendable rod called the notochord, with hints of vertebra-like elements beside it. It had no jaws and no paired limbs.

Is Haikouichthys really a vertebrate? Scientists still argue about it. Some call it a vertebrate; the latest analyses place it on the “stem,” among the relatives that branched off just before the true vertebrates. Either way, it shows the body plan we inherited: a head with eyes and gills, a muscular body that bends side to side, and a rod down the back.

For a living comparison, look at the lamprey, a jawless fish shaped like an eel. A lamprey has a notochord and only small arches of cartilage along it, not a true backbone, which is why it is a good model of the early stages. Its relative, the hagfish, has a skull but little more than rudiments of vertebrae. Lampreys and hagfish are the only jawless fish alive today, and molecular data show that they are each other’s closest relatives.

What does a backbone do for an animal? A rod that bends but does not shorten lets the muscles pull one side and then the other, which is how a fish swims. A string of small bones is both strong and flexible. And because it sits inside, it grows with the body. A trilobite, with its armor on the outside, has to shed that shell to get bigger, which makes it helpless for a while; a vertebrate never has to. Over the next 500 million years that inside skeleton would hold up bodies from hummingbirds to blue whales.

The invention of the bite

For the first 80 million years or so, as far as the fossils show, vertebrates had no jaws. These armored “jawless fish” were suction feeders and filter feeders, straining or sucking up mud for the food in it. Others were parasites. The lamprey is the model here too: its mouth is a round sucker lined with small teeth, and a sea lamprey fastens onto a living fish, rasps through its skin, and drinks its blood.

The second turning point was a hinge. Jaws are bones that swing against each other, and they did not come from nowhere. Jawless fish had a row of bony or cartilage arches that held their gills open. In the ancestors of jawed fish, the front arch bent into a hinged pair of bars, an upper and a lower, and became the jaw. The next arch back became a brace for it. Your own jaw is the result of that rebuilding of a gill support.

The oldest evidence for this comes, again, largely from China. In 2011 Gai Zhikun and colleagues described Shuyu (曙鱼 Shǔyú, “dawn fish”), a jawless fish from Zhejiang about 430 million years old, whose nostrils, pituitary duct, and brain case were already laid out the way they are in jawed fish: the groundwork for jaws, before the jaws themselves. Then came fish with the jaws themselves. A newly found fossil site in Chongqing, about 436 million years old, has produced small fish with complete bodies and jaws, including Xiushanosteus, and a site in Guizhou has produced the scales and spines of a shark relative, Fanjingshania, about 439 million years old. Entelognathus (全颌鱼 Quánhéyú, “whole-jaw fish”) from Qujing, Yunnan, about 420 million years old, was an armored fish whose upper and lower jaws were lined with the same kinds of bones found in bony fish and in humans. In that sense it gave scientists a first look at the face of our distant jawed ancestors.

These are the earliest jawed fish known, and the dates keep moving back. That is how it goes with fossils: the earliest one found is never the earliest one there was. What the finds show is that jaws arose in the Silurian period, about 440 million years ago or a little before, and that by the Devonian, from about 419 million years ago, jawed fish had spread into every sea and river. The Devonian is called the Age of Fishes.

The best-known jawed fish of that age is Dunkleosteus (邓氏鱼 Dèngshìyú), an armored predator that lived in the sea over what is now Ohio, around 360 to 380 million years ago. Its jaws carried sharp bony blades instead of teeth, and could shear through the armor of other fish. Older popular accounts gave it a length of up to ten meters. A 2023 study, which built a better method for estimating body length from fossil heads, put typical adults at about 3.4 meters: shorter than once thought, but stout, about as heavy as a great white shark more than a meter longer.

What did jaws change? Eating first. An animal with jaws can grab, hold, and cut, so it can hunt moving prey and eat things that are larger than itself, instead of filtering mud. Jaws are also tools for defense, for fighting rivals, and, in many animals, for carrying, digging, and building. The jaw you feel when you chew or talk is the hinge that began in a gill. And as Lesson 3 will show, a piece of it moved again: two of the three tiny bones in your middle ear began as jaw-joint bones.