Geographical Distribution of the Boab

Australian Boab

There are only eight species of Adansonia known throughout the world.  Six are indigenous to Madagascar, with one species native to mainland Africa and the Arabian Peninsula, where they are known as the Baobab, and one in Australia – our Boab.  Adansonia gregorii, the Australian Boab tree, is found only in the Kimberley and western Victoria Region of the Northern Territory. 

Of the eight known Adansonia species, A. gregorii and A. digitata (from mainland Africa) are very closely related, both visually and biologically, compared to the six Adansonia species native to Madagascar.   Why is the Australian Boab restricted to the Kimberley and Western Victoria River Region, yet it’s counterpart in mainland Africa has a much wider distribution?  Well the answer might be in how it reached Australia! 

There are three possible theories as to how the Boab came to Australia: 

Species Evolution prior to the break-up of Western Gondwana

The genetic differences between A. digitata and A. gregorii are too small for the species to have evolved prior to the break-up of Africa, Madagascar, India, and Australia over 100 million years ago.  The differences between these species and that of the Madagascan species indicate that they evolved after the break-up of the Western Gondwana land-mass.

Transoceanic Dispersal

So did the Boab reach Australian shores by floating as seed pods across the Indian Ocean?

Boab Seed Pods and seeds

This theory is questionable because the shell of the A. gregorii seed pod is the thinnest of all the Adansonia species which makes it unlikely to have survived floating across the ocean before getting waterlogged.  

Secondly, the direction of present-day oceanic currents are unlikely to have resulted in seed pods reaching the north-west coast of Australia, although current directions may have been different in the past.  

Thirdly, if it did float from Africa, why is not established more broadly in regions across the northern part of Australia? 

Was the Boab brought to Australia by human migration?

The very close genetic connection between the mainland Africa A. digitata and the Australian A. gregorii, compared to the six Adansonia species native to Madagascar, raises the possibility that dispersal of the Boab occurred with human migrations out of Africa which commenced 60,000 to 70,000 years ago. 

Adding support to this interesting scenario is that the geographical distribution of A. gregorii is very similar to that of the ancient Australian rock art known as the Bradshaw Rock Art (recently renamed as “Gwion Gwion”).  This style of rock art was named after Joseph Bradshaw who was the first European to record it in the 1890s.

The origin of the Bradshaw Rock Art is hotly debated because they are so strikingly different to rock art found anywhere else in Australia, and appear to be older than the Wandjina Rock Art that is also present in the Kimberley region. 

There are a number of interesting features that lend support to this link between the arrival of the Boab in Australia and the Bradshaw Rock Art:

• The Bradshaw Rock Art, with their delicate painting techniques and mysterious images, are remarkably similar to distinctive African cultures;
•  Boabs feature frequently in the rock art, suggesting a link that goes beyond that which might be expected if the tree’s usefulness was discovered after arriving in Australia;
• The rock art images show detailed scenes from daily life with accurate depiction of fauna, and include images of large boats carrying up to 30 passengers.  These boats have high prows suggesting possible open-ocean capability;
• The pulp of Boab/Baobab nuts is high in vitamin C and the seeds are very nutritious.  The pods keep easily for over a year, making them a very useful item to take on a long sea journey.

To the present-day African Hadzabe culture, the Baobab represents an important source of food and water.  Their ancestors survived the catastrophic Toba event that wiped out most of the world’s human population at that time.  Toba, a supervolcano in Sumatra, erupted about 74,000 years ago and plunged the earth into an instant 1,000-year ice age.  The survival of hunter-gatherers, such as the Hadza, may well have depended on their intimate knowledge and close relationship with a bountiful tree such as the Baobab.

To learn more about the Boab and the Australian Aborigines, please click here

References