The ancestor of all hammerhead sharks probably appeared abruptly in Earth's oceans about 20 million years ago and was as big as some contemporary hammerheads
But once the hammerhead evolved, it underwent divergent evolution in different directions, with some species becoming larger, some smaller, and the distinctive hammer-like head of the fish changing in size and shape, said CU-Boulder Professor Andrew Martin of the ecology and evolutionary biology department.
In the new study, scientists focused on the DNA of eight species of hammerhead sharks to build family "gene trees" going back thousands to millions of generations.
In addition to showing that small hammerheads evolved from a large ancestor, the team showed that the "signature" cephalofoils of hammerheads underwent divergent evolution in different lineages over time, likely due to selective environmental pressures, said Martin.
The team used both mitochondrial DNA passed from mother to offspring and nuclear DNA -- which is commonly used in forensic identification -- to track gene mutations. The researchers targeted four mitochondrial genes and three nuclear genes, which they amplified and sequenced for the study.
"These techniques allowed us to see the whole organism evolving through time," Martin said. "Our study indicates the big hammerheads probably evolved into smaller hammerheads, and that smaller hammerheads evolved independently twice."
The researchers sampled hammerheads from across the globe -- including the waters of the southeast United States now under siege by the Gulf oil spill -- as well as Australia, Panama, Hawaii, Trinidad and South Africa. Most of the hammerhead DNA was obtained at local markets, where the peddling of sharks and other fish is common practice.
The team sequenced the DNA of the sharks, constructing a "phylogenetic" tree that shows how all of the species are related and when each species originated, said Martin. The hammerhead ancestor probably lived in the Miocene epoch about 20 million years ago.