It is the world's most pressing scientific puzzle, but experts warn there may never be conclusive answers over the source of the coronavirus, after an investigative effort marked from the start by disarray, Chinese secrecy and international rancour. 

January 11 marks the anniversary of China confirming its first death from Covid-19, a 61-year-old man who was a regular at the now-notorious Wuhan wet market.

Nearly two million deaths later, the pandemic is out of control across much of the world, leaving tens of millions ill, a pulverised global economy and recriminations flying between nations.

Yet China, which has broadly controlled the pandemic on its soil, is still frustrating independent attempts to trace the virus' origins and the central question of how it jumped from animals to humans.

There is little dispute that the virus which brought the world to its knees sparked its first known outbreak in late 2019 at a wet market in the central Chinese city of Wuhan where wildlife was sold as food, and the pathogen is believed to have originated in an undetermined bat species.

But the trail ends there, clouded by a mishmash of subsequent clues that suggest its origins may predate Wuhan as well as conspiracy theories -- amplified by US President Donald Trump -- that it leaked from a Wuhan lab.

Establishing the source is vital for extinguishing future outbreaks early, leading virologists say, providing clues that can guide policy decisions on whether to cull animal populations, quarantine affected persons, or limit wildlife hunting and other human-animal interactions.

China won early kudos for reporting the virus and releasing its gene sequence in a timely manner, compared with its cover-up of the 2002-03 SARS outbreak.

But there has also been secrecy and shifting stories on Covid-19.

Daniel Lucey, a Georgetown University epidemiologist who closely tracks global outbreaks, said the Wuhan market might not even be the issue. He notes that the virus was already spreading rapidly in Wuhan by December 2019, indicating that it was in circulation much earlier.

That's because it may take months or even years for a virus to develop the necessary mutations to become highly contagious among humans.

Findings that the virus may have existed in Europe and Brazil before Wuhan's outbreak, added to the confusion.

And it has kept foreign experts at arm's length, with a planned mission by World Health Organization virus sleuths now in limbo after China denied them entry. On Saturday, China reversed the decision.

What the scientists will be allowed to see or may expect to find a year on is also in doubt.

Some researchers remain hopeful that the origin may still be found. But others are skeptical.

Diana Bell, a wildlife disease expert at the University of East Anglia who has studied the SARS virus, Ebola and other pathogens, said focusing on a particular origin species is misguided.

She says the overarching threat has already been exposed: a global wildlife trade that fosters a "combustible mix" of trafficked species, a known breeding ground for disease outbreaks.

"We need to stop the wildlife trade for human consumption."