Baobab

 

It’s sometimes called the “Tree of Life” for its supportive qualities and long lifespan stretching thousands of years—and it’s also known as the “Upside-Down Tree” for its root-like branches. But no matter the name, the baobab tree has long captivated the human imagination with its surreal shape and enduring presence.

Eight living species of the baobab genus Adansonia exist worldwide: one in mainland Africa, six in Madagascar and one in northwestern Australia. Yet, the origins of these botanical behemoths have eluded scientists for years.

Many researchers thought the trees began on the African mainland, then spread to the other locations, the New York Times’ Rachel Nuwer reports. However, a study published last week in the journal Nature used the genomes of each baobab species to unravel the tree’s ancient origins, instead tracing its lineage to Madagascar 21 million years ago.

Over the course of millions of years, the team reports, diverse baobab species emerged across Madagascar, driven by ecological competition and environmental conditions—including altitude, sea level and volcanic activity. Eventually, two baobab species traveled from Madagascar to continental Africa and northwestern Australia where they, too, evolved into unique species.

 

These baobab seeds were likely transported across continents by the Indian Ocean gyre, a system of rotating currents in the Indian Ocean that circulate clockwise.

“The plants almost certainly got to Africa and Australia floating on or with vegetation rafts,” study co-author Tao Wan, a botanist at the Wuhan Botanical Garden in China, tells Reuters’ Will Dunham. Vegetation rafts, or naturally floating mats of plant material and dirt, may have helped other species move across continents, such as early primates that got from Africa to South America.

“We were delighted to be involved in this project uncovering patterns of baobab speciation in Madagascar followed by the astonishing long-distance dispersal of two species, one to Africa and another to Australia,” study co-author Andrew Leitch, a plant geneticist at Queen Mary University of London, says in a statement.

As diverse baobab species evolved, they developed a mutually beneficial relationship with several animals that acted as pollinators, such as bats, hawk moths, lemurs and bush babies, which may have influenced the different tree species’ flower structures.

Additionally, researchers determined that lower sea levels allowed baobab trees to better thrive across Madagascar.

“During times of relatively low sea level, vast areas of Madagascar were probably suitable for baobab population expansion and dispersal, whereas periods of high sea level led to smaller suitable areas, population fragmentation, species isolation and reduced gene flow,” the study explains.

Scientists say these findings could shed light on modern threats to baobabs. Now, with global sea-level rise accelerating, the baobab tree’s ability to expand and survive could severely decrease.

“The lowered chance of expansion coupled with the distinct ecological niches baobabs occupy is a recipe for a threatened population,” per a Chinese Academy of Sciences statement about the study. “Then, adding in habitat loss of the trees themselves along with their pollinators like fruit bats and hawkmoths, and conservation of baobabs becomes a pressing issue.”

During a previous study that examined African baobab trees between 2005 and 2017, researchers reported that nine of the 13 oldest and five of the six largest baobab trees on the continent died during that time. The team called it an “event of an unprecedented magnitude” and suggested that southern Africa’s changing climate may be to blame, but more research is needed for confirmation, Nature News’ Sarah Wild wrote at the time.

 

Additionally, two baobab species in Madagascar currently show low genetic diversity, potentially hindering their ability to adapt to climate change. A third species on the island is at risk of extinction due to interbreeding with a more widespread relative, reports the New York Times.

These three species are categorized as endangered on the International Union for Conservation of Nature’s Red List, and the other baobab species on Madagascar are listed as least concern. However, given their declining populations, more conservation measures may be required, per the study. The authors propose a re-evaluation of the trees’ conservation statuses and recommend that some get an upgrade to higher threat levels.

Data on baobabs from the new study “will inform their conservation to safeguard their future,” study co-author Ilia Leitch, a plant genomics researcher at Royal Botanic Gardens, Kew, in England, tells BBC News’ Helen Briggs.

“This work has uncovered new insights into the patterns of speciation in baobabs and shows how climate change has influenced baobab distribution and speciation patterns over millions of years,” she adds in the statement.

Aaron Boorstein for The Smithsonian magazine.