NEW YORK (NYTIMES) - The animal kingdom is one of life's great success stories - a collection of millions of species that swim, burrow, run and fly across the planet.
All that diversity, from ladybugs to killer whales, evolved from a common ancestor that likely lived more than 650 million years ago.
No one has found a fossil of the ur-animal, so we can't say for sure what it looked like. But two scientists in Britain have done the next best thing. They've reconstructed its genome.
Their study, published in Nature Communications, offers an important clue to how the animal kingdom arose: with an evolutionary burst of new genes.
These may have played a crucial part in transforming our single-celled ancestors into creatures with complex bodies made of many kinds of cells.
The new genes also proved to be remarkably durable. Of all the genes in the human genome, 55 per cent were already present in the first animal.
"The big surprise was how many of them there were," said Dr Jordi Paps, an evolutionary biologist at the University of Essex and co-author of the new study.
Dr Paps and Dr Peter W.H. Holland, a zoologist at the University of Oxford, began by drawing an animal family tree.
Our own species belongs to one branch, the vertebrates (animals with spines), along with birds, reptiles and fish.
Genetic studies have shown that our closest invertebrate relatives include creatures like starfish, while jellyfish and sponges are among our most distant cousins.
Researchers have also identified the single-celled species that are the closest relatives to the animal kingdom - tiny aquatic protozoans that prey on bacteria.
From all these tree branches, the scientists picked 62 species, including our own, to study closely.
They searched the DNA of the organisms, cataloguing all the genes that encode proteins, the molecules that carry out countless chemical reactions in our bodies and give it structure. (Humans have about 20,000 protein-coding genes.)
Dr Paps and Dr Holland toted up nearly 1.5 million genes in all, and then estimated when they first evolved. It makes for a wide-ranging genetic history.
Humans and sharks, for example, make haemoglobin using nearly identical genes.
That means haemoglobin genes were already present in their common ancestor.
But haemoglobin genes can't be found in more distantly related animal species, such as sponges. So the gene evolved in early vertebrates - long after the origin of the animal kingdom.
While some genes like haemoglobin are young, others are old. The researchers found 6,331 genes that were present in the common ancestor of all living animals.
Many of those genes appeared long before animals themselves. Some are essential to the basic workings of all living things - such as copying DNA - and first evolved billions of years ago.
Other genes arose more recently and can be found today in our close single-celled relatives.
These findings confirmed earlier studies, which had been carried out on fewer species. When animals arose, evolution gave old genes new jobs.
Single-celled protozoans, for example, use some genes to make proteins that let them cling together in tiny colonies.
In animals, these genes helped cells to glue themselves permanently together - a requirement for building a body.
But 1,189 of the genes in the ancestral animal can't be found in our closest known single-celled relatives. These new genes must have evolved in proto-animals.
Dr Paps said there were at least two ways for that to have happened. Sometimes a random string of DNA with no function mutates and begins producing a protein.
Alternatively, an existing gene may be accidentally duplicated. One copy may accumulate mutations until it produces a new kind of protein, even as the other copy keeps doing the original gene's original job.
The newly acquired DNA turns out not to be involved in a random assortment of jobs. Instead, many of these genes play crucial roles in building and running animal bodies - for example, making proteins that cells use to send signals to other cells.
Dr Paps and Dr Holland also found that a number of the genes developed by proto-animals are implicated in cancer.
Many of these genes keep cells working together harmoniously, and when they mutate, cells may multiply out of control.
Dr Iñaki Ruiz-Trillo, a biologist at the Instituto de Biología Evolutiva in Barcelona, Spain, who was not involved in the new study, said that the flowering of new genes might represent a fundamental characteristic of the animal kingdom.
They "could be used to define what an animal is", he said.
Dr Paps speculated that the burst of new genes might have appeared in early animals because the environment somehow triggered many mutations.
But another possibility is that proto-animals gradually accumulated all these new genes over hundreds of millions of years - a stretch of evolutionary history that scientists cannot yet document by studying living species.