Here's a rundown of some of 2018's most eye-catching stories.

A "safe" limit for warming

A rise in global temperatures of 2C by the end of this century has long been seen as the gateway to dangerous climate change. Researchers had argued that keeping within this limit was necessary to avoid the most damaging effects of global warming.

But some have been pushing for an even lower target of 1.5C. In October, climate scientists released a major report detailing what would be involved in keeping the temperature rise to within that tougher limit.

This would result in millions fewer people losing their homes to rising seas, fewer species at risk of extinction and a drastic reduction in the numbers of people experiencing water scarcity.

But it would also be hugely expensive and require "rapid, far-reaching and unprecedented changes" to society. The report didn't tell governments what to do, but set out a range of approaches including heavy cuts in greenhouse gas emissions, a rapid transition to renewable energy and lifestyle and dietary changes.

The earliest animals

The one-million-plus animal species alive today are staggeringly diverse, from the giant oceanic blue whale to the wriggly earthworms beneath our feet. But their early evolution from single-celled ancestors remains shrouded in mystery.

In the hunt for the earliest animal life, much attention has been focused on a group of enigmatic life forms - known as the "Ediacaran biota" - from more than 500 million years ago. These were some of the first complex organisms to appear on Earth.

But their position on the tree of life is difficult decipher. These curious creatures have been variously categorised as lichens, fungi, and even as a halfway house between plants and animals.

In September, scientists were able to extract molecules of cholesterol from a fossilised Ediacaran life form called Dickinsonia, which resembled a flat jellyfish. Cholesterol is one of the molecular hallmarks of animal life, clearly demonstrating that the Ediacaran biota were animals.

Giant plastic 'berg

The world's plastic waste crisis was one of the big themes of 2018. The problem had been highlighted by the BBC's Blue Planet 2 series, presented by David Attenborough, which contained shocking footage of the devastation wrought on the oceans and marine life by our addiction to plastic.

In April, our Science Editor, David Shukman, visited Indonesia to report on a morass of plastic waste that had clogged rivers and canals in Bandung, on the Indonesian island of Java. The crisis was so acute, the army was called in to help clean up a vast plastic 'berg of bottles, bags and other plastic packaging.

Worryingly, the problem only looks set to get worse. In March, a report commissioned by the UK government suggested the amount of plastic in the ocean could triple in a decade unless litter is curbed.

Ghost particle busters

Neutrinos are some of the fundamental building blocks of the Universe. These sub-atomic particles hurtle around the cosmos more-or-less unimpeded, interacting with very little. In fact, it's estimated that a single neutrino particle can pass through a light-year (about 10 trillion km) of lead without hitting a single atom.

Many neutrinos we encounter on Earth come from the Sun or the Earth's atmosphere. But the origins of one group of ultra-high-energy neutrinos remained mysterious until this year. In July, an international team traced one of them to a distant galaxy firing a particle "ray" straight at the Earth.

This type of galaxy is called a blazar. It has an intensely bright core caused by the energy of its central, massive black hole. As matter falls into the hole, enormous jets of charged particles emerge, turning these galaxies into vast particle accelerators.

The IceCube experiment in Antarctica has been collecting data on these ultra-high-energy neutrinos for six years, but this was the first time researchers were able to match them with a source in the sky.

A watery Mars - and Moon

We know there is water on Mars in the form of ice, and there are possible signs of occasional liquid flows. But in July, a team of scientists reported the discovery of a 20km-wide lake sitting under the planet's south polar ice cap.

Nasa's Curiosity rover has been exploring the rock remnants of an ancient lake bed, but this is the first sign of a persistent body of water today. The result was exciting because scientists have long hunted for signs of present-day liquid water on Mars.

"We are not closer to actually detecting life," said Manish Patel, from the UK's Open University, "but what this finding does is give us the location of where to look on Mars."

Mars wasn't the only cosmic body making watery headlines. In August, researchers published what they said was the most definitive evidence yet for ice on the surface of the Moon.

Data from India's Chandrayaan-1 spacecraft suggests the presence of icy deposits at the north and south poles. This ancient water might be accessible as a resource for future human missions to the Moon.

What happened to the Stonehenge builders?

The field of ancient DNA - which involves extracting and analysing genetic material from long-dead people - has provided us with unprecedented insights into the past. One striking result from 2018 was the discovery that the ancient people of Britain were almost completely replaced in a mass migration from the continent about 4,500 years ago.

The Neolithic Britons had just erected the big stones at Stonehenge when they were overrun by newcomers known as the Beaker people. This resulted in 90% of the British gene pool being replaced in just a few hundred years. Why this happened is unknown. But disease, famine and conflict are all potential candidates.

In a different study released in 2018, researchers showed that 50,000-year-old bone fragments from Russia belonged to girl who was half Denisovan and half Neanderthal. The Denisovans and Neanderthals were distinct species of human who inhabited Eurasia before our species - Homo sapiens - left Africa.

Icy impact

In November, scientists identified what looked to be a large impact crater under the Greenland ice. The 31km-wide depression came to light when scientists examined radar images of the island's bedrock.

The bowl was probably excavated by a 1.5km-wide iron asteroid sometime between about 12,000 and three million years ago. Some researchers have their doubts about the evidence presented so far. But it has raised some intriguing possibilities, including a potential connection to a period of strong cooling that punctuated the climatic warming seen as the Earth emerged from the height of the last Ice Age.

There is a longstanding hypothesis that this dip in temperatures could have been the result of the sun's rays being blocked by debris thrown into the atmosphere by an impact and the smoke and ash from wildfires it set off. If further work confirms the age of the crater is close to the lower end of the age range, it could re-ignite interest in this old debate.

An earlier exodus

Multiple lines of evidence suggest that the ancestors of most humans living outside Africa left the continent in one migration 60,000 years ago. But there is some evidence that pioneer modern humans (Homo sapiens) made forays outside Africa before this time.

In January, scientists unveiled the jawbone of a modern human who died in Israel 185,000 years ago, tens of thousands of years earlier than previous evidence. Received wisdom suggests that these earlier excursions failed to provide modern humans with a permanent foothold in Eurasia.

But the jawbone fits in with the emerging picture of earlier out-of-Africa migrations that spread further into Eurasia than many had believed. These pioneers appear to have lived alongside other human species such as the Neanderthals and Denisovans. But it remains a mystery as to why their genetic signatures are not preserved in people alive today.

Rocks from Mars

After years of discussion and one false start, the European and American space agencies made their first significant move towards bringing back rocks from Mars.

In April, Nasa and Esa signed a letter of intent that would lead to the first "round trip" to another planet.

The venture would allow scientists to start answering key questions about Martian history, including whether the planet once hosted life. But it would also allow geologists to start building an accurate chronology for events in Martian history.

US missions over the last few decades have contributed enormously to our understanding of the Red Planet in situ, but there are mass constraints on the experiments that can fit on a payload destined for Mars.

There's no comparison to the information scientists will be able to glean from studying Martian rocks and soil with the scientific instruments available in terrestrial laboratories.

Plastic in our water

Plastic waste is increasingly pervasive in our daily lives, and this extends to our drinking water. Research by the journalism organisation Orb Media found an average of 10 plastic particles per litre in major brands of bottled water.

In the largest investigation of its kind, 250 bottles bought in nine different countries were examined. Nearly all of them contained tiny plastic particles.

Our planet's far north is often regarded as a pristine wilderness. But this year, researchers voiced their concern about the large concentrations of plastic building up in Arctic sea-ice.

The number of particles in just one litre of melted Arctic sea ice was found to be higher than in the open ocean. Scientists said there was a need for more research into its effects on zooplankton, invertebrates, fish, seabirds, and mammals.

Courtesy: BBC