By comparing genome similarity across species, biologists can piece together which species are closely related, and which are not. However, when an organism dies its DNA can quite quickly break down except for in very unusual circumstances. In the film Jurassic Park, dinosaur DNA is extracted from insects preserved in amber, but even when locked up in hardened tree sap, DNA cannot last anywhere near the 66 million years it would need to survive from the Cretaceous-ending meteor impact until now for us to genetically sequence a dinosaur. The oldest DNA yet found was beneath the Greenland icecap and is about 2 million years old. Because most DNA degrades quite quickly after death, we can only hypothesize about the size, structure and make-up of the genomes of T. rex, the first mammals and the common ancestor we shared with MK-D1 billions of years ago. We may be able to assert, reasonably confidently, that an extinct species had a particular gene if descendants from a shared ancestor with that species alive today all have the gene, but that only takes us so far.
DNA has allowed biologists to piece together the tree of life in astonishing detail, but it is of less use in working out what extinct species looked like. Even if we can assume an extinct species had a particular gene, that does not mean we know with certainty how the gene influenced the species’ development or phenotypic traits. The same gene can often be co-opted for multiple different uses. For example, the protein GroEL is found in many bacteria species, and it plays a key role in ensuring that chains of amino acid fold into the correct protein shape. The same protein, produced by exactly the same gene, is also used as an insect toxin by antlions.
The larvae of a type of lacewing are known as antlions, or doodlebugs. These insect larvae dig cone-shaped pits in sand, partially burying themselves at the bottom. Unsuspecting insects, often ants but sometimes beetles, lose their footing on the edge of the sandpit and slide to its base, where the antlion injects them with a paralysing toxin before consuming their prey’s innards and discarding the inedible exoskeleton. The toxin is produced by bacteria that live in the antlion’s salivary gland, and it is the same protein used by bacteria to fold proteins. Many, and perhaps the majority of genes, produce proteins that are put to more than one use, and that makes it hard to know how an extinct species might have used a particular gene or which phenotypic traits it helped produce.
Different species also switch on genes for different lengths of time during development. Fish species that grow proportionally larger fins turn on some genes for longer as they are growing their fins than species that grow smaller ones. Many differences in body shape and organ sizes between mice, monkeys, humans and other mammals are due to different species switching genes on and off at different points in development, rather than because they are using different genes. A gene associated with brain development consequently may not tell you much about how big a brain an extinct organism might have had. We need to turn to fossils to understand the form of life past.
People have been aware of fossils for at least two and a half thousand years. The ancient Greeks realized that fossilized shells were once living shellfish, and when Xenophanes of Colophon uncovered such fossils, he correctly surmised that the land on which he had found them must once have been under the sea. Although many cultures from lands as far afield as China and the Middle East were aware of fossils and appreciated that they were records of long-dead organisms, it was not until the 1840s, when Richard Owen, the founder of London’s Natural History Museum, coined the name ‘dinosaur’ that interest in fossils became mainstream. In retrospect, it became obvious that humans had stumbled across dinosaur fossils for millennia, but had attributed them to mythical monsters, including giants. Richard Owen’s breakthrough was to analyse bones from three different species of dinosaur and realize they all came from an extinct group of reptiles that could grow to enormous size. Dinosaur translates to ‘terrible lizard’, and once the public appreciated that monstrous beasts really did once walk the Earth, our fascination with dinosaurs began.
Dinosaur fossils are reasonably common, although the vast majority of dinosaur corpses rotted away leaving no trace. Their fossils are common because dinosaurs were incredibly successful, roaming the planet for 165 million years, and some species would have had large populations. Big dinosaurs also had large bones and teeth, and it is the hard parts of animals and plants that are most likely to fossilize. In comparison, individual cells or soft tissues such as muscles, brains and flowers rarely form fossils. Nonetheless, in some cases softer tissues have been preserved. Despite this, there has been a lot of life on our planet over the last 4 billion years that we know little about, even though palaeontologists have got very good at predicting where best to look for fossils from each past geological epoch.
It is not always straightforward to tell whether an animal is an adult of a small species or a juvenile of a larger one. As species develop, their body shape can change radically, and when only a few bones or teeth are fossilized, interpreting them can be hard. Juveniles and adults of a species can differ substantially in both form and size. Emu chicks are hatched at about 5 inches tall, about the size of the world’s smallest flightless bird, and are fluffy and remarkably cute. Yet they grow to a height of five feet seven, with the adults being monstrous and bearing little resemblance to the young.
A few years ago, while visiting my father-in-law’s farm in Queensland, we were brought a day-old emu chick that had become separated from its father. Emus, and other ratites such as ostriches, are unusual for birds in that it is the father that does all the parental care of the chicks. Young birds of most species are not as attractive as the adults, being born without feathers. Emus, chickens and ducks are different, in that their young do have feathers, and the emu chick we were given to care for was cute. And if we didn’t look after it, it would die.
I originally named the emu Emma, but on pondering whether it might in fact be male decided we needed a gender-neutral name. Emma-Steve imprinted on my father-in-law, and he on it, and he raised it on his farm. For months the bird was a beloved pet, and it turned out it would eat pretty much anything, including the keys to the tractor, but it was most partial to steak. Most emus eat a diet less protein-rich than Emma-Steve’s, but given the cattle on the farm, steak was not in short supply. By the time Emma-Steve became an irascible ‘teenager’ at about a year of age, it was the biggest emu we had ever seen, growing to about six feet tall. If we hadn’t known what emu chicks looked like, we would have been in for a nasty surprise.
It turned out that Emma-Steve was a him, and he developed from an extraordinarily cute chick into a violent beast with a strong desire to top the dominance hierarchy. After Emma-Steve chased my daughter Sophie into the house, attacked my father-in-law and generally deterred all visitors, it was decided it was time to take action. He underwent what biologists refer to as a soft release, which meant we left the gate open before shutting it once he’d left. But we would regularly see him around. His large size did him no harm, and on one sighting we were excited to see him with a clutch of eight young.