A passion for rock art led to the identification of a strange tree in the Kimberley which turned out to be Africa’s newest species. It’s an extraordinary story of scientific discovery that could be the beginning of something much bigger.
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If trees could be charismatic, then a boab in Australia, or a baobab in Africa and Madagascar, are those trees. They’re like a caricature of a normal tree with swollen trunks and gnarly branches. There’s just one species of Australian boab, six species of Madagascan baobab, and until recently just one species of African baobab.
Tim Willing is a naturalist and author who has worked in the Kimberley for over three decades. His life’s work has resulted in new plant species being described including a wattle named for him, but one strange boab growing amongst weeds on the edge of Broome eluded identification.
“I really didn’t know what it was,” he says. “As most Kimberley residents know, the boab tree has flowers that sit upright… This one has flowers that hang down, almost a bit like a little lampshade.”
As a passionate, self-taught botanist with decades of experience working with relatively unstudied flora in the Kimberley, there are few people better placed to identify unusual plants than Tim Willing. But despite all his own knowledge, as well as being well connected with botanists and herbariums around the country, the identity of the strange boab remained a mystery.
Rock art connection
And then in late 2012 the answer came from a most unexpected quarter. Professor Jack Pettigrew is a retired neuroscientist based in Queensland. Since retiring, Professor Pettigrew has worked intently on his new passion of Kimberley rock art.
“So I got hooked on studying Bradshaw Art as a kind of retirement project, and the first step I had to settle were the boabs,” he says.
Professor Pettigrew had been struck by the way the conspicuous boab trees seemed to mirror the distribution of the intriguing rock art known as Bradshaw or Gwion Gwion art. The Gwion Gwion rock art, like the boab, is only found through parts of the Kimberley, and is strikingly different to other Australian rock art in being finely detailed and depicting figures with unusual headdresses, tassels and poses.
Ever since the first European description of Gwion Gwion figures by pastoralist Joseph Bradshaw in 1891, it has commonly been supposed that it must have been influenced by a visiting foreign culture; Bradshaw himself compared it to Egyptian art. But the apparent association between boabs and the rock art suggested a more southern African influence to Professor Pettigrew, where the boabs relatives grow today.
The most common theory is that the boab’s ancestor must have floated across the Indian Ocean from Africa to take root in North Western Australia. But Professor Pettigrew is not alone in seeing problems with this idea.
“We know from elephant bird eggs that it takes two years on the currents to get here. So that’s a bit of a problem because we know that boab nuts can’t withstand two years, they get waterlogged,” he says.
Professor Pettigrew also points out that if boabs did float to Australia, they should have been able to spread along the coast where they can happily be cultivated.
“So why were they in such a narrow spot, where that narrow spot was also the same narrow spot where you find Bradshaw Art?”
Out of Africa
Professor Pettigrew theorised that this was no coincidence, and like boabs, Gwion Gwion art must have come from Africa. It’s a radical theory with little support from Australia’s archaeological community.
The age of the rock art has been controversial with some of the oldest measured ages being up to 18,000 years old. But the discovery of Gwion Gwion art associated with paintings of megafauna thought to have gone extinct over 40,000 years ago suggests it may be much older.
But even this controversial date is much younger than the amount of time thought to be needed for a boab to evolve into the modern Australian species. It’s presumed they must have been evolving in Australia for hundreds of thousands, if not millions of years before the Gwion Gwion figures were painted, but no one really knows for sure.
“So we had to decide how to set about determining the age of the boabs in the Kimberley,” says Professor Pettigrew.
As a biologist, the Professor took a genetic approach to figure out how long boabs had been in Australia. He hovered in a helicopter above 220 boabs across the Kimberley and plucked leaves from each one.
“Sometimes I’d grab hold of the little branch with the leaves on it and give it a pull and the cables wouldn’t cut it. And so the whole chopper that was hovering was shaking,” the Professor recalls with a chuckle.
By looking at how many different kinds of a particular boab gene which was known to mutate at a predictable rate, Professor Pettigrew calculated that the boab was a much more recent arrival to Australia than previously thought.
“The results are a bit surprising. We’re still writing it up because when you’re working with material that tells you that the age is so young, namely around 70,000 years, you want to check it. So we’ve used a few different techniques, most of them agree, but one of them doesn’t agree,” he says.
If correct, this age brings the boab tantalisingly closer to the age of Gwion Gwion art. But another problem facing the Professor’s radical theory was baobob chromosomes. Although the African baobab has the most similar genes to the Australian boab, the African tree has them packaged into twice as many chromosomes (called tetraploid) than the boab (called diploid), making it less likely to be a recent ancestor.
“It’s a lot of hand-waving to say the African baobab has transformed into a diploid,” says Professor Pettigrew.
A new species
And so the Professor theorised that there must be another baobab.
“Maybe the diploid is still there in Africa and it hasn’t been noticed, it escaped attention.”
Not being the type to simply wonder, he was off to Africa to find the lost ancestor of Australian boabs. Professor Pettigrew has made significant discoveries and ruffled feathers with radical theories in his chosen career of neuroscience, but to drive into the heart of Africa in search of an undiscovered species of well-known tree could only be described as audacious.
“So you’re up there on the Tanzanian Plateau,” Professor Pettigrew recounts. “And then I have a portable microscope…and I set it up on the bonnet of the Landcruiser, stick the leaf-lift in, look in, hey presto. ‘Look it’s a diploid!’”
The discovery was followed with taxonomic descriptions of the flower and pollen grains for the publication of a new species description naming the tree Adansonia kilima, or the African mountain baobab in a scientific journal in late 2012. It was this eureka-moment for Professor Pettigrew in Africa that led to another eureka-moment for Tim Willing in Broome.
“When I saw the photographs of the flowers I recognised it straight way… I thought for one thing that Jack Pettigrew would never believe that we already had it growing in Broome and that it was already flowering,” says Mr Willing.
The explanation for how the newly described African ancestor of Australian boabs came to be growing on the outskirts of Broome has got nothing to do with ancient cultures or evolution, and everything to do with Broome’s old government nursery.
“In those days they grew lots of plants and distributed them from the nursery and one of them was the African baobab,” says Mr Willing. “They got some seed from Zimbabwe or South Africa in the 1980s and sowed it assuming it was the normal African baobab.”
It seems that just by chance, some of the seeds were actually the then undescribed African mountain baobab and the adult trees are now growing in the Kimberley.
African long-shot
As well as solving the mystery of the identity of the strange boab in Broome, Professor Pettigrew says it brings him a step closer to proving an African connection to Gwion Gwion rock art. He’s now working on a genetic technique to age the paintings to try and show that they were created at the same time he proposes that boabs arrived in Australia.
“The Bradshaw art is alive… I’m trying to learn as much as I can about the microbes that live in the art. I’ve characterised the fungus…and I can tell you what the relative age of the art is from the fungus. The trouble is, it’s a new species of fungus. It has its nearest relative in Antarctica. And so I don’t know its mutation rate,” says Professor Pettigrew.
He says that if he can calibrate his DNA evidence he can prove the African connection to the Australian rock art.
“If the boabs arrived 70,000 years ago, and the rock art is 70,000 years old, it’s a hell of a coincidence… That would be enough for me, but of course some sceptics might say ‘Well you know, it might have been spacemen that might have brought the boab down,’” says Professor Pettigrew.
June Ross is an Adjunct Professor of Archaeology at the University of New England and she is sceptical about Professor Pettigrew’s conclusions about Gwion Gwion rock art.
“What you’ve got to look at is; ‘Is this a likely fit with what we already know?’ There is no evidence for humans being in Australia 70,000 years ago, and there is no evidence of any similar art in Africa anything like that old,” she says.
While Professor Pettigrew may have discovered a new baobab species, linking Africa to Kimberley rock art is a leap well beyond what Professor Ross says can actually be proven. But Professor Pettigrew insists that his theory can’t be disproven.
“Sometimes you find something that absolutely contradicts your hypothesis and then you’ve got to accept that. But at the moment there’s nothing like that. At the moment the pieces are gradually being put together that support this idea.”