Solenodons are some of Earth’s strangest creatures. Venomous, nocturnal and insectivorous, they secrete toxins through their front teeth – an unusual habit for a mammal. More to the point, the planet’s two remaining species – the Cuban and the Hispaniolan solenodon, both highly endangered – have endured, virtually unchanged, for the past 76 million years. Other related species have become extinct.

And that makes solenodons very important, according to Professor Sam Turvey, of the Zoological Society of London. “They are the last fruits on an entire branch of the tree of evolution,” said Turvey, who was last month awarded one of the most prestigious awards in zoology, the Linnean medal, for his work on evolution and human impacts on wildlife. “There are no close counterparts to solenodons left on Earth, yet they have been on the planet since the time of the dinosaurs.”

Solenodons have been brought close to extinction by the mongoose, the carnivore introduced to their native islands to kill snakes and rodents. They are classic examples of an “Edge” – evolutionary distinct and globally endangered – species. This means they have no close relatives and represent our last chances to preserve entire branches or trunks of the evolutionary tree.

Other examples include the critically endangered vaquita, a species of porpoise from the Gulf of California, and the Sumatran rhino. “A lot of attention is paid to other threatened rhino species,” said Turvey, “but the Sumatran, which is down to only a few dozen survivors, is the only woolly rhino left on the planet. It is special.”

These animals stand in contrast with other threatened species which have close relatives that fill similar ecological niches. The polar bear, for example, is closely related to the grizzly. Should the former die out, the latter could provide a fair amount of genetic substitution, say scientists. By contrast, there is no species that could do the same for solenodons.

Another example is provided by the gibbons of Asia. Most species are now endangered, with some, such as the Hainan gibbon, facing imminent extinction. Yet gibbons were once widespread and common. “In the tombs of ancient Chinese emperors,” said Turvey, “we have discovered bones of gibbons, kept as high-status pets, from species previously unknown to science. Over millennia, we have lost countless species, and entire genera, of gibbons in Asia.”

Turvey’s particular period of evolutionary interest is the past 10,000 years. “Before that time, the climate was changing rapidly as the world emerged from the last ice age. However, it then stabilised and that means we can be more sure that the extinctions which occurred from then on were caused by humans.”

In that period, humans conquered the continents and around 250 species of mammal were wiped out as we made our way round the globe, with the Caribbean suffering particularly badly. “More that 100 species of monkey, sloth, rodent and bat were killed off from its islands, and humans were undoubtedly to blame for most of these extinctions,” said Turvey.

“Many of the rodents would have been cat-sized and would have played an important role in dispersal of seeds and vegetation, but would also have been an important source of protein for newly arrived humans. As a result, they were wiped out. Virtually every island in the tropics lost a unique species of rodent thanks to humans.”

In addition, hundreds, possibly thousands, of species of bird were lost, particularly in the tropical Pacific, he said. Most of this carnage was caused by the hunting of birds – often flightless like the dodo – and mammals for food. “That pattern has changed recently,” added Turvey. “It is not direct killing that is doing the main damage today; it is our destruction of wild habitats to make room for farms or roads. That has become the main cause of extinctions.”

Animals on the edge

Evolutionarily distinct and globally endangered (Edge) animals have a highly threatened status but few closely related species. Apart from solenodons, examples include:

• Kitti’s hog-nosed bat, the world’s smallest bat, found in caves in Thailand, is also the world’s smallest mammal. It is the only remaining member of the Craseonycteridae family and is also endangered through habitat disruption.

• The wild Bactrian camel is critically endangered and is found on the steppes of China and southern Mongolia. Only about 1,400 individuals survive.

• The Chinese giant salamander is the world’s largest amphibian, and can grow to almost two metres in length. It is critically endangered through habitat loss and pollution – and because it is used in traditional Chinese medicine.