Mayflies live a day, humans live a century, if we're lucky, but what is the oldest living organism on the planet? For scientists, accurately proving the age of any long-lived species is a hard task.
Under the boughs of a 300-year-old sweet chestnut tree in the Royal Botanic Gardens in Kew, Tony Kirkham, head of the arboretum, confirms that trees are capable of outliving animals.
Proving this can involve some traditional detective work, as he explains: "First of all we can look at previous records, to find out if a tree was growing there at a set date. Then we look at paintings and artwork, to look to see if that tree was present. And old ordinance survey maps quite clearly show ancient trees, especially important ones."
A well-known way of measuring the age of a tree is by counting the rings in its trunk: one ring per year of growth. It's a process known as dendrochronology and only works for certain types of tree that have an annual growth spurt.
The obvious problem is that counting rings normally involves cutting down the tree.
Arboriculturalists get around this by using an increment borer, a drill that allows them to take out a core, and count the rings without fatally damaging the tree.
It's a delicate art, and, Tony says, back in the 1960s, one scientist's drill broke off inside the bristlecone pine tree he was sampling.
The kit is expensive, and to help him recover the lost instrument, a forester helpfully cut down the tree. Once felled, the tree could be easily aged, and was found to be 5000 years old.
"It was terrible but so much science came out of that opportunity, and since then, we've found trees that are as old, if not older," admits Tony.
A team of researchers in the US keeps a list, called the Old List, of officially dated ancient trees.
They've found a sacred fig tree in Sri Lanka that is at least 2,222 years old.
There's a Patagonian cypress tree in Chile which, at 3,627 years old, is as old as Stonehenge.
A Great Basin bristlecone pine in California's White Mountains named Methuselah comes in at 4,850 years old. But the oldest tree on the list, an unnamed bristlecone pine from the same location, has a core suggesting it is 5,067 years old.
This time-worn tree has lived through the rise and fall of the Roman Empire. It was already established when the Ancient Egyptians started building pyramids.
Is this 5,000-year-old Great Basin bristlecone pine the oldest single living thing on the planet? That depends on your definition of a "single tree".
In Fishlake National Park in Utah in the US lives a quaking aspen tree that most people would struggle to see as "a tree".
It's a clonal tree called "Pando", from the Latin meaning "I spread", and for good reason.
It is so large that it is easy to mistake for a forest. However, Pando, despite being the size of Vatican City, has all sprung from one seed, and, over the years, has grown a single vast rootstock supporting an estimated 50,000 tree trunks. Accurately estimating how many years is problematic, says population geneticist Prof Karen Mock from Utah State University, who works on the aspen.
"There have been all kinds of different estimates but the original tree is almost certainly not there," he told the BBC.
Clonal trees grow in all directions and regenerate themselves as they go. This means taking a core from a trunk will not give you the age of the whole tree.
Scientists try to get around this problem by equating size to age. It's an inaccurate process and Pando's estimated age ranges from a few thousand to 80,000 years old.
Prof Mock hopes that a new technique, looking at how many DNA mutations are accumulated over time, could give them another way of assessing the age of this remarkable tree.
If a tree can live to 80,000 years old, why stop there?
Is any organism on Earth immortal? No, according to ageing expert Joao Pedro de Magalhaes from the University of Liverpool.
"All organisms can die, so there's no immortal species per se," he says.
"You do have complex vertebrate species that appear not to age, like the Galapagos tortoise or a cave-dwelling salamander called the olm. I do say 'appear not to age'; we haven't studied any of them for 500 years. It's hard enough to get grants for five-year projects," he jokes.
How long you live depends partly on your place in the world; your ecological niche.
The organisms at the top of the food chain have very few predators, so are likely to live longer and to pass down that trait through generations.
Cold weather also plays a part: Antarctic glass sponges take the title of 'oldest living animal', with an estimated life span of 15,000 years.
"We don't know for sure," says Dr Magalhaes, "because, of course, nobody was there to check them 15,000 years ago."
The estimates are prone to error but according to Dr Magalhaes, sponges in Antarctica do grow very slowly because of the cold, which fits the model of slow-growing creatures having longer life spans.
However, the oldest, precisely measured organism living on Earth today remains, for now, a Great Basin Bristlecone pine tree. Pando the quaking aspen and Antarctic glass sponges could be much older but their ages are assumed from indirect measurements and educated guesswork.
Tantalisingly, Prof Mock and Tony Kirkham agree that there's still more to hunt for; the actual oldest living organism on the planet almost certainly has not yet been found.
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