Genetic clocks weren’t created equal: they tick at different speeds. Since the first genome of an organism—the bacterium, Haemophilus influenza—was sequenced 20 years ago, the clocks of a menagerie of species have been estimated, including lampreys, mice, opossums, and humans have also been measured. By comparing the degree of genetic difference among species and their common ancestor, scientists decipher how fast (or slow) their DNA has changed over millions of years. Can you guess whose clock ticks the fastest?
—Research and text by Alex Riley

Quiz: Please click to choose which evolves faster from the two choices.
1) Sea Lamprey vs. Chicken
Chicken
Gallus gallus domesticus
Tuatara
Sphenodon punctatus punctatus
Tuatara
Sphenodon punctatus punctatus
Human
Homo sapien
Human
Homo sapien
African Clawed Frog
Xenopus laevis
African Clawed Frog
Xenopus laevis
Sea Lamprey
Petromyzon marinus
Sea Lamprey
Petromyzon marinus
Grey, Short-tailed Opossum
Monodelphis domestica
Grey, Short-tailed Opossum
Monodelphis domestica
Coelacanth
Latimeria chalumnae
Coelacanth
Latimeria chalumnae
Mouse
Mus musculus
Mouse
Mus musculus
Elephant Shark
Callorhinchus milii
Elephant Shark
Callorhinchus milii
Chicken
Gallus gallus domesticus
Answers (from slowest to fastest evolution):
1. Elephant Shark
Callorhinchus milii

The elephant shark has the slowest genetic clock known, beating the coelacanth, the previous record-holder. The genome study that clocked the speed also revealed why chimeras, sharks, and rays have cartilage skeletons rather than bone: they lack SCPP, genes crucial for its production.

2. Coelacanth
Latimeria chalumnae

Along with a having a relatively slow-ticking genetic clock, the coelacanth—whose genome was sequenced in 2013—has substantial portions of its genome containing active regions of so-called “jumping genes,” a common source of evolutionary innovation.

3. Sea Lamprey
Petromyzon marinus

The oldest representative of living animals with backbones, the genome of the sea lamprey is crucial to understanding the earliest stages of vertebrate evolution.

4. Chicken
Gallus gallus domesticus

The first agricultural animal to have its genome sequenced also provided tantalizing glimpses into bird (and dinosaur) evolution. Due to the possibility of in ovo (in the egg) experiments, chicken embryos are a common favorite among developmental biologists.

5. Grey, Short-tailed Opossum
Monodelphis domestica

The opossum became the first marsupial to have its genome sequenced in 2007, revealing a substantial distinction between marsupials and other (placental) mammals: the latter have far more “jumping genes” (retroviruses) dotted around their genomes.

6. Human
Homo sapien

Just like mice, under 1% of our genome encodes proteins; and research suggests we could even do without 20% of it. But others argue that this “junk DNA” is much more pervasive. Consider the onion: despite our complexity, it has five times more DNA.

7. Mouse
Mus musculus

The archetypal research animal’s genome was published in 2002—a year after the first human genome. With a large amount of conserved genes between these two genomes, biomedical studies into mice can shed light on our own genetic disorders and diseases.

8. African Clawed Frog
Xenopus laevis

This frog, Xenopus, can regenerate its limbs, tail, and other body parts during certain life stages, making the genome of this amphibian—the first to have its genome sequenced—a vital and extensive tool for developmental biologists, providing insights into its 350 million year evolution.

9. Tuatara
Sphenodon punctatus punctatus

This reptile, endemic to New Zealand, is a Triassic Period relic. However, modern and ancient DNA show that the tuatara may have one of the fastest vertebrates clocks. Richard Fortey, from the London Natural History Museum, says morphology need not keep pace with genomic change.