There have been a few times in the history of mankind when we nearly died out as a species. Anthropologists call these events “bottlenecks”, times when the population of humans shrank – perhaps to as few as 2,000 people over 50,000 years ago. At those levels, we would be categorised as an endangered species on the IUCN Red List, existing in even fewer numbers than wild tigers do today. We survived, and in fact we’ve thrived, mainly because we adapted our environment to suit our needs. But to what degree has the survival and triumph of our species changed our planet? The best people to answer this could well be those who have the grandest perspective. Geologists can take a 4.5-billion-year step back and look at the human impact on our planet in the context of its long history – they can identify changes in the rock record within layers of deposited sediments that build up and are compressed over time. There have been plenty of dramatic changes to our planet in the past: this spinning lump of rock has flipped from a “Snowball” Earth to searing temperatures devoid of ice; there have been times when life has flourished, and times when it has been beset by mass extinctions. Big changes in planetary states show up in the rock record as stripes that reveal the sudden disappearance or appearance of certain fossil species, or climatic changes that reveal changes in past sea level.

Red List counts 'on the brink' species

East Asia's status as the world's main "extinction hotspot" is confirmed in the new Red List of Threatened Species.

Sawfish snout senses, swipes and stabs

The spectacular snouts of sawfish are

revealed as complete hunting weapons, sensing prey and killing them.

The saws, which can grow more than a metre long in some species, have

previously been identified as able to sense prey by their electric fields.

Now, researchers have filmed the fish impaling prey on the teeth of the

saws. They suggest in Current Biology that

sawfish are more active hunters than previously thought, which could help in their much-needed conservation.

All seven species are listed as Critically Endangered on

the internationally-recognised Red List.

The researchers, mainly based in Australia, suggest sawfish may be unique

among their peers in possessing a snout, or rostrum, that works both as a

sensory organ and a hunting weapon.

South Africa troops tackle rhino poachers

Rhinos are killed for their horns in many parts of Africa

South Africa is to deploy hundreds of extra troops along its borders to help fight gangs smuggling rhino horns, the government has announced.

Justice Minister Jeff Radebe said four military companies would be sent to the borders with Zimbabwe, Swaziland and Lesotho.

Syndicates export the horns from Africa to parts of Asia and the Middle East.

In 2011, a record 450 rhinos were killed by poachers in South Africa, the Department of Environment Affairs says.

Mr Radebe said about 600 soldiers would join the fight against rhino poaching.

"The deployment includes army engineers who are conducting repairs and maintenance on the Zimbabwe-Mozambique border fence, which is approximately 140 kilometres (85 miles)," Mr Radebe said.

Last year troops were deployed along the Mozambican border, many of them inside the world-famous Kruger National Park where more than 200 rhinos were killed last year.

"Effective border management is part of the government crime prevention strategy, which assists to deal with cross-border crime syndicates and curb poaching," said Mr Radebe.