2015: The year in science and environment
While astronomers homed in on habitable worlds beyond our Solar System, negotiators in Paris were very much focused on preserving our own bit of real estate in the cosmos. The BBC's Paul Rincon looks at the year in science and environment news.
World in motion
This year's Paris summit was billed as a last-ditch chance for world leaders to find a way to prevent dangerous climate change.
In the end, the deal struck in the French capital was partly legally binding, partly voluntary. Many participants agreed the outcome was less than perfect. But the crucial part had been done: uniting all the world's nations in a single agreement on tackling climate change for the first time in history.
The process sets out a clear long-term plan for keeping global temperature rise under 2C (the threshold for dangerous warming). But - importantly - it also incorporates a review process allowing ambition to be increased in the future.
As WWF-UK's chief executive David Nussbaum commented: "Paris is just the starting gun for the race towards a low-carbon future."
Brit in space
When Tim Peake was selected as a European Space Agency astronaut in 2009, the UK paid virtually nothing towards human spaceflight. Had this state of affairs continued, the Chichester native might have been left watching from the sidelines as, one-by-one, his classmates flew to the space station. But the government was persuaded to change its policy and, consequently, Tim got his flight on 15 December.
It's almost 25 years since Britain had an astronaut, Helen Sharman, who flew to the space station Mir under a privately sponsored programme. In the intervening years, the International Space Station was built and flights to and from the orbiting outpost have appeared to become so routine that they rarely merit much news copy in the UK.
The launch from Baikonur Cosmodrome has reinvigorated interest in the working life of the orbiting outpost, at least while there's a Brit aboard. But officials behind Tim's mission will hope that the "Tim effect" will persist longer than his six-month sojourn - and even inspire a new generation of budding spacefarers.
When Nasa's New Horizons spacecraft was launched in 2006, Pluto still held full planetary status. By the time the probe finally flew by its target in July, carrying the ashes of Pluto discoverer Clyde Tombaugh, it had been relegated to a lesser category of object.
While the arguments rumble on, none of this detracted from the mind-blowing spectacle of seeing the last of the classical nine planets up close for the first time. By all accounts, Pluto turned out to be a much more dynamic place than anyone could have imagined this deep-frozen world would be.
Planet or not, Pluto - as mission scientists were fond of saying - did not disappoint.
A home from home?
In the quarter of a century since the discovery of the first planet beyond our Solar System, a trickle of discoveries has turned into a deluge.
This year we were treated to a world about 60% larger than Earth, but which circles a Sun-like star at a very similar distance to our own, suggesting that conditions might be suitable for liquid water. This is important, of course, because liquid water is considered a prerequisite for life.
How Earth-like a planet is deemed to be depends to some extent on the properties one chooses - and it's not known for certain that Kepler-452b is rocky, rather than gaseous.
But, on the other hand, most previous potentially habitable planets are around so-called red dwarf stars, which are cooler than our own. That means these worlds need to be located much closer to their parent star to receive the same level of heating. It also means they are exposed to bombardment by charged particles from their suns - which might prove hostile to biology.
A new branch on the tree
The discovery in a South African cave system of 15 partial skeletons belonging to a new human species caused a sensation in 2015.
They assigned the individuals, which possess a mixture of primitive and modern features, to the grouping Homo - which includes us.
The researchers were not able to date the remains of Homo naledi, but think they could be about three million years old. That would place them in a crucial period where more ape-like creatures evolved bigger brains, a more human-like body plan and, with it, the capabilities to begin mastering their environment.
In another significant discovery, researchers identified the remains of modern humans in China that proved to be at least 80,000 years old. This was a big surprise, since genetics and archaeology both point to a dispersal of Homo sapiens from Africa no earlier than 60,000 years ago.
The discovery creates a problem for the most widely accepted theory about the settlement of Earth by modern humans. Were these Chinese settlers an advance party that later went extinct? Or could the human story be more complicated than we imagined? As geneticist Pontus Skoglund said: "We are just starting to cope with this data point."
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