Humans are defined as members of the species Homo sapiens; human beings. Humans also refer to a member of any of the extinct species of the genus Homo, such as Homo erectus or Homo habilis, that are considered ancestral or closely related to modern humans.

Mud in reservoir: Levels in the Cantareira reservoir system remain dangerously low. Fires to clear land for agriculture in Sao Felix Do Xingu municipality, Para, Brazil - June 20, 2009. In Brazil's biggest city, a record dry season and ever-increasing demand for water has led to a punishing drought. It has actually been raining quite heavily over the last few days in and around Sao Paulo but it has barely made a drop of difference. The main reservoir system that feeds this immense city is still dangerously low, and it would take months of intense, heavy rainfall for water levels to return to anything like normal. So how does a country that produces an estimated 12% of the world's fresh water end up with a chronic shortage of this most essential resource - in its biggest and most economically important city? It's interesting to note that both the local state government and the federal government have been slow to acknowledge there is a crisis, despite overwhelming evidence to the contrary. That might have been a politically expedient position to take during the recent election campaign, when the shortage of water in Sao Paulo was a thorny political issue, but the apparent lack of urgency in the city and wider state now is worrying many. At the main Cantareira reservoir system, which feeds much of this city's insatiable demand for water, things have almost reached rock bottom. Huge pipes suck out what water remains as the reservoir dips below 10% of its usual capacity.

Climate change: Soot's role underestimated

The burning of wood is a major source of black carbon the world over.

Black carbon, or soot, is making a much larger contribution to global warming than previously recognised, according to research.

Scientists say that particles from diesel engines and wood burning could be having twice as much warming effect as assessed in past estimates. They say it ranks second only to carbon dioxide as the most important climate-warming agent. The research is in the Journal of Geophysical Research-Atmospheres. Black carbon aerosols have been known to warm the atmosphere for many years by absorbing sunlight. They also speed the melting of ice and snow. This new study concludes the dark particles are having a warming effect approximately two thirds that of carbon dioxide, and greater than methane.

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.