Of the many people who made history in 2019, some surprised themselves and the world by emerging from obscurity to make their mark, though one remains anonymous for the time being - "The Whistleblower" behind the impeachment probe into US President Donald Trump.
Following are brief profiles of eight history-makers in politics, climate and humanitarian activism, music and astronomy who were unknown quantities in 2018.
Trump impeachment 'Whistleblower'
Although huge efforts have been made to expose him, the person whose complaint threatens to bring down the president of the United States is still known only as "The Whistleblower".
Reliably reported to be a mid-level, male CIA analyst in his early 30s who specialises in Eastern European issues and previously worked in the White House, he filed an anonymous complaint in August charging that Donald Trump pressured Ukraine counterpart Volodymyr Zelensky to help find dirt on his Democratic rivals - a violation of US laws against seeking foreign help in US elections.
It was a finely written, nine-page memo describing specific Trump actions, and while it was based on secondary sources - his colleagues in the intelligence and diplomatic communities - first-hand witnesses have corroborated what he said, and more, in the months since it surfaced.
By sending his complaint to the inspector general for the US intelligence community, The Whistleblower set in motion a series of reviews and then news articles that quickly snowballed into the House impeachment probe that may see Trump put on trial in the Senate in the new year.
Many whistleblowers stay anonymous, and some collect million-dollar rewards for exposing fraud.
But this one will not gain a reward and likely will not remain unknown. Conservatives have already circulated a name and photograph online.
Republicans in Congress have tried to expose him, alleging he is a Democrat out to get Trump.
But the impeachment process he sparked now fuels itself, meaning that, outed or not, his impact will long be felt in Washington politics.
Greta Thunberg, 16, climate activist
What started as humble protest has turned Greta Thunberg into the world's green conscience and the voice of a generation's frustration with inaction on climate change.
It all started in August 2018 when Thunberg decided to skip school and sit outside Sweden's parliament, holding a sign reading "school strike for the climate".
Within months her struggle gained worldwide attention and the shy 16-year-old - with her piercing eyes and trademark braids - found herself addressing world leaders at the World Economic Forum in Davos and at the European Parliament.
Young people from around the world began staging their own school strikes, and the "Fridays for Future" movement was born.
Following her ethos of avoiding air travel, she crossed the Atlantic on a zero-emission sailboat to attend a UN climate summit in New York in September.
The Stockholm-born teenager's eyes brimmed with tears and her voice cracked with emotion as she delivered a fiery speech to world leaders.
"How dare you?" she thundered.
"You have stolen my dreams and my childhood with your empty words."
Thunberg, the daughter of an opera singer mother and an actor-turned-producer father, has also faced severe criticism and been subjected to a swarm of online conspiracy theories.
Some have mocked her youth, called her a puppet of doomsayers or tried to discredit her because of her Asperger's syndrome, a diagnosis she has never hidden.
But no one can deny that the passionate climate activist's struggle has helped put climate change back at the top of the agenda.
A survey published by the European Commission in April found that six in 10 Europeans thought "climate change is one of the most serious problems facing the world," an increase of 17 percentage points compared with 2017.
Hong Kong students
Brandishing Molotov cocktails, black-clad Hong Kong students cast off their bookish, meek image in 2019 to become a global symbol of democratic resistance in the face of unyielding authoritarian power.
Crowds have marched peacefully for greater freedoms since Britain's handover to China in 1997. But this year saw pent-up anger explode in ways once unimaginable for a financial hub that has always prided itself on stability and safety.
What started as a popular protest over a proposed bill allowing extraditions to mainland China morphed into a popular anti-Beijing revolt and turned the city’s tourist shopping districts into brick-strewn urban battlegrounds.
The typical frontline protester is university educated and under 30. Arrest and injury figures suggest around a third of those who have taken to the streets are women.
Gas masks with bright pink filters - dubbed "pig snouts" in Cantonese - became ubiquitous as people sought to conceal their identities.
In a year that saw major unrest in the streets of Catalonia, Chile and Venezuela, the David and Goliath image of the leaderless protest movement in Hong Kong caught the global imagination, triggering messages of solidarity from around the world and statements of support from the EU, UN and United States.
Embracing the slogan "Liberate Hong Kong, revolution of our time", protesters have battled with riot police, stormed the local legislature and spray-painted their core issues and demands across the city, shredding the notion of a peaceful transition to complete Chinese control in 2047 envisaged in the 50-year "one country, two systems" deal.
Close to 6,000 protesters have been arrested with nearly 1,000 charged. But the crowds keep coming and there is little sign the movement's more moderate supporters are going to abandon frontliners they have dubbed "the braves".
Faced with regular warnings from Beijing of disastrous consequences should the unrest continue, the protesters have responded with their now omnipresent chant: "If we burn, you burn with us."
Venezuela's Juan Guaido
For a long time he didn't distinguish himself as an outspoken critic of President Nicolas Maduro, but when he proclaimed himself president in January, Juan Guaido suddenly emerged as the socialist leader's main opponent.
A key challenge now is to continue to inspire a wilting opposition.
When he burst on the scene in January, the 36-year-old lawmaker initially energised a weakened opposition whose key leaders were imprisoned, exiled or in hiding.
On January 23, a few days after taking the helm as speaker of parliament, the only state institution controlled by the opposition, Guaido proclaimed himself acting president, declaring Maduro's re-election illegitimate.
He was swiftly recognised by the United States and about 50 other countries. His popularity rating among Venezuelans soared to 63%. By October, however, it had dropped more than 20 points.
A trained industrial engineer, Guaido said he has "tried everything" to push Maduro out of office, as the country endures a deep economic crisis that has driven 3.6 million people to flee since 2016.
With great fanfare on February 23, Guaido tried to break a border blockade to bring stockpiled international food and medical aid into the country, calling on the military to abandon Maduro. The gambit failed.
On April 30, a military uprising won support of only a handful of officers and was quickly subdued by the government.
Guaido, married with a two-year-old daughter, describes himself as a survivor of the "Vargas tragedy" - a December 1999 landslide in the northern coastal state where he lived with his mother and five siblings, which killed thousands.
The Venezuelan prosecutor's office, considered a branch of government by the opposition, has filed a number of lawsuits against him, for which he could face up to 30 years in prison.
But Washington has repeatedly warned Caracas that jailing Guaido would be Maduro's "last mistake".
On January 5, his term as parliament speaker officially ends. Agreements between political groupings could allow him to remain in the post, despite a drop in his ability to inspire mass protests, most Venezuelans having abandoned the street to focus on the daily business of survival.
Revolutionary 'icon' Ala Saleh of Sudan
Ala Saleh, dressed in traditional white Sudanese garb and standing atop a car, became the symbol of Sudan's uprising as she led chants against the now-ousted autocrat Omar al-Bashir in April.
Saleh, 22, was propelled to internet fame after a photograph of her with one hand raised in the air singing and cheering along with crowds of protesters went viral, earning her the moniker of "Kandaka", or Nubian queen.
An engineering student, Saleh grew up in a middle-class Sudanese family in Khartoum and was relatively unknown until her photograph went viral during the anti-Bashir protests.
But since earlier this year, she has become a voice for women's rights in the northeast African country, where centuries of patriarchal traditions and decades of strict laws under the former regime have severely restricted the role of women in Sudanese society.
"The existing discrimination and inequality women face, coupled with conflict and violence over decades, has resulted in women being subjected to a wide range of human rights violations, including sexual and gender-based violence on an epic scale," Saleh said at an open debate at the UN Security Council last month.
She told the UNSC that even wearing trousers or meeting male friends took courage as it was criminalised under the former regime.
During Bashir's 30-year rule, authorities enforced a strict public order law that activists said primarily targeted women, through harsh interpretations of Islamic sharia law.
On November 26, the country's new transitional cabinet led by Prime Minister Abdalla Hamdok scrapped the law, although the ruling sovereign council has yet to ratify the move.
Saleh has faced criticism for attracting global attention even as many female activists faced brutal punishments during Bashir's rule.
But many defend her rise to fame.
"She was a normal person like all others who took to the streets against the former regime," said activist Khalid Tabidi.
Migrants activist Carola Rackete
German Carola Rackete was the captain of a migrant rescue ship in the Mediterranean who became a left-wing hero in Italy for challenging then far-right interior minister Matteo Salvini's "closed ports" policy.
The dreadlocked Rackete, 31, was skipper of the Sea-Watch 3, one of several ships used by international charities to aid migrants attempting the perilous sea journey from North Africa to Europe on rickety boats.
On June 12, Rackete's ship picked up 53 migrants adrift aboard an inflatable raft off the coast of Libya.
The Italian authorities allowed some of the migrants to be taken in for health reasons but refused entry to 43 others, leading to a two-week stand-off at sea.
As conditions on board worsened, Rackete eventually sailed her ship to the island of Lampedusa despite an order from Italian officials not to dock there.
She was arrested on June 29, although a judge overturned that order on July 2, saying she had acted "out of necessity" because of the migrants' condition.
Italy's highest court is set to rule in January on whether Rackete's arrest was warranted.
Salvini described the incident, in which Sea-Watch 3 allegedly hit a police speedboat, as an "act of war" and referred to Rackete as "the German criminal".
During a visit to Italy in November to present her book "The World We Want", Rackete was given a police escort after coming under attack from anti-migrant groups.
But she said the harassment "does not really affect me".
"On the contrary, I am now more sensitive to the racism that some people suffer and to the discrimination and social justice that exist in the world," Italian media quoted her as telling supporters at the book launch.
The fairytale rise of Lil Nas X
A little over a year ago, Montero Hill had dropped out of university, was living with his sister, had no job, car or even a driver's licence.
Today - thanks to record-breaking single "Old Town Road" - he is the millionaire country-rap superstar known as Lil Nas X.
The song topped the Billboard Hot 100 for 19 consecutive weeks between April and August, breaking a record dating to the mid-1990s when Mariah Carey and Boyz II Men spent 16 weeks at number one. Luis Fonsi and Daddy Yankee with Justin Bieber also topped the charts for 16 weeks with "Despacito" in 2017.
All versions of "Old Town Road" have been played more than 1.3 billion times on streaming site Spotify.
Lil Nas X, 20, composed the song based on a beat he purchased for $30 from a Dutch record producer.
The result merged thumping bass and rap with a twangy banjo sound more associated with country music.
Billboard barred the song from its rankings of country music songs, arguing that it did not have enough elements of that genre to merit inclusion.
A few days later, Lil Nas X released a remix of what was already a hit, starring country star Billy Ray Cyrus, father of pop star Miley Cyrus.
Even with the added legitimacy of Billy Cyrus, a two-time Grammy nominee in country categories, the remix was also left out of the country rankings.
Both versions went on to become number one in the main Billboard charts, catapulting the previously unknown artist into the celebrity stratosphere.
Lil Nas X has also seduced fans with his down-to-earth personality and humour. He has never hesitated to don traditional country clothing, including cowboy hat, jacket and boots.
After successfully achieving a rare marriage of rap and country tunes, the young artist shook the hip-hop world in early July by announcing that he is gay.
Although some female rappers had already come out, such as Young M.A, Lil Nas X became the first prominent male rapper to do so.
It was a significant development for an industry that, while less macho than in the past, tends to present a more traditional side of masculinity.
The woman who photographed a black hole
US computer scientist Katie Bouman became an overnight sensation in April for her role in developing a computer algorithm that allowed researchers to take the world's first image of a black hole.
The 30-year-old, currently an assistant professor at the California Institute of Technology (Cal Tech), was a member of the Event Horizon Telescope (EHT) Collaboration when the team captured the image.
Bouman said she first began working on the EHT as a graduate student studying computer vision at MIT and found that black hole imaging shared striking similarities with work she had done on brain imaging based on limited data from an MRI scanner.
The EHT Collaboration had spent more than a decade building an Earth-sized computational telescope that combined signals received by various telescopes working in pairs around the world.
However, since there were a limited number of locations, the telescopes were able to capture only some light frequencies, leaving large gaps in information.
In 2016, Bouman developed an algorithm named CHIRP to sift through the true mountain of data and fill in the gaps, producing an image.
While the images were captured in 2017, the final result had to be independently validated by four EHT teams working around the world to avoid shared human bias.
On April 10, 2019, a final image was released - a moment that Bouman, then a postdoctoral fellow at the Harvard Smithsonian Center for Astrophysics, described as "truly amazing and one of my life's happiest memories".
Testifying before Congress in May about her research, Bouman praised her team that included several early-career scientists - like herself - whose work had been vital to the project.
"Like black holes, many early-career scientists with significant contributions often go unseen," she said.
But that's not the case with her anymore.