After more than 20 million years on the planet, the Yangtze river dolphin is today officially declared extinct, the first species of cetacean (whale, dolphin or porpoise) to be driven from this planet by human activity.

An intensive six-week search by an international team of marine biologists involving two boats that ploughed up and down the world's busiest river last December failed to find a single specimen.

Today, the scientific report of that expedition, published in the peer-reviewed journal of the Royal Society, Biology Letters, confirms the dolphin known as the baiji or white-fin in Chinese and celebrated for its pale skin and distinctive long snout, has disappeared.

To blame for its demise is the increasing number of container ships that use the Yangtze, as well as the fishermen whose nets became an inadvertent hazard.

This is no ordinary extinction of the kind that occurs frequently in a world of millions of still-evolving species. The Yangtze freshwater dolphin was a remarkable creature that separated from all other species so many millions of years ago, and had become so distinct, that it qualified as a mammal family in its own right. It is the first large vertebrate to have become extinct for 50 years and only the fourth entire mammal family to disappear since the time of Columbus, when Europeans began their colonisation of the world.

The three previous mammal families gone from the face of the Earth are the giant lemurs of Madagascar, which were eliminated in the 17th century, the island shrews of the West Indies, probably wiped out by the rats that accompanied Colombus on his voyage, and the Tasmanian tiger, the last known specimen of which died in captivity in 1936. (The most famous creature to have become extinct in the past 500 years, the Dodo, was a bird.)

Sam Turvey, conservation biologist at the Zoological Society of London, who led the expedition to find the Yangtze dolphin and is chief author of the paper, said: "The loss of such a unique and charismatic species is a shocking tragedy. This extinction represents the disappearance of a complete branch of the evolutionary tree of life and emphasises we have yet to take full responsibility in our role as guardians of the planet."

Several other species are "just hanging on" in the Yangtze and could disappear within a few years unless action is taken now, Dr Turvey warned. They include the Chinese alligator, the finless porpoise and the Chinese paddlefish, which grows up to 7m long but has not been seen since 2003.

"There is a lot of interest now in the baiji - but it has come too late. Why does no one pay attention to a species until there are none left? We really have to use the baiji as a wake-up call to act immediately to prevent it happening again.

"What is poignant is that the Yangtze is a fast river system with a unique range of endemic species. Once they are lost there, they are lost everywhere," he said.

The object of last December's expedition was to rescue any baiji found and remove them to a 21km-long oxbow lake in the nature reserve of Tian'ezhou for an intensive breeding programme. Each of the two boats operated independently with scientists scanning the water with binoculars - dolphins have to surface to breathe - and listening with hyprophones for the distinctive whistles. Despite the technology, they found nothing.

"We used a very intensive survey technique. Both of the boats counted the same number of porpoises - we saw everything that was there. We didn't see a single dolphin," Dr Turvey said.

The cause of the freshwater dolphin's demise was instead all too plain to the investigators. It had become a victim of the world's most populous country's race to get richer. One tenth of the world's population live in the Yangtze river basin. During the expedition, scientists counted 19,830 ships on the 1,669km of the river they surveyed - one large freight vessel every 800m.

The Yangtze dolphin navigated by sonar - its eyes are useless in the murky water - but in a motorway jammed with container ships, coal barges and speed boats, its sonar was deafened and it ran a high risk of being hit or torn by propellers.

An even greater threat came from the nets and 1,000m lines of hooks used by fishermen.

Although they did not intend to catch dolphins, the creatures became entangled in the nets or lacerated by the bare hooks - almost half of all dead baiji found in the past few decades have died in this way. In addition, pollution had fouled their natural habitat and completion of the Three Gorges Dam worsened the decline in smaller fish on which the baiji fed.

The last mammal families to become extinct

Island shrews

Extinct: 1500

The West Indian "island shrews" or nesophontids are known only from sub-fossil remains. They were about the size of a rat and died out following the accidental introduction of black rats, with which they could not compete, from European ships. They were the most ancient land mammals of the West Indies and their extinction represented the loss of an entire mammalian order.

Giant lemurs

Extinct: 1650

The giant lemurs of Madagascar weighed up to 180lb, more than a silverback gorilla. They died out as a result of hunting by humans.

Tasmanian Tiger

Extinct: 1936

The Tasmanian tiger, or thylacine, left, looked like a large striped dog, with a wolf's head and heavy tail. It was actually a marsupial, related to the kangaroo, with a pouch to raise its young. European settlers feared it and killed it whenever they could. Thylacines never bred in captivity - the last known one dying in Hobart zoo on 7 September 1936.