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Australia: The Land Where Time Began |
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Ancient Human Influences on the Evolution of Baobab Trees and
Distribution in Australia – New Genetic and Linguistic Analysis
This study investigates the role of human agency in the gene flow and
geographical distribution of the Australian Baobab,
Adansonia gregorii. The
genus
Adansonia is an iconic
genus that is endemic to Africa, Madagascar and northwest Australia
(Baum, 1995; Pettigrew et al.,
1995) that has been valued by humans for its many uses. In Africa the
distribution of genetic variation in baobabs has been partially
attributed to dispersal that humans mediated over thousands of years,
though this relationship has never been investigated for the Australian
species. Rangan combined linguistic and genetic data in order to analyse
geographic patterns of gene flow and movement of word-forms for
A. gregorii in the
Aboriginal languages of northwest Australia. Weak geographical structure
and high gene flow was shown by comprehensive assessment of genetic
diversity. Congruence of gene flow patterns and directional movement of
Aboriginal loan words for
A. gregorii was
demonstrated by genetic-linguistic analysis. Along with previous
archaeobotanical evidence from the Late Pleistocene and Holocene, these
findings suggest that ancient humans influenced significantly the
geographic distribution in northwest Australia of
A. gregorii.
The role humans played in the shaping of crop diversity has always been
considered an integral factor of the evolution of agriculture in
different regions of the world (Donohue & Denham, 2009; Perrier et
al., 2011; Roullier, Benoit,
McKey & Lebot, 2013). Combining genetics, linguistics and archaeobotany
with interdisciplinary research has enhanced further the understanding
of the geographic patterns of the domestication of animals and crops and
subsequent diffusion of humans (Donohue & Denham, 2009; Perrier et
al., 2011; Roullier, Benoit,
McKey & Lebot, 2013). Yet, very little comparable research has been
carried out on how the evolution and distribution of uncultivated plants
that have, nonetheless, had a long history of use by humans has been
influenced by anthropogenic agency (Blench, 2007; Smith, 2011; Bostoen &
Grollemund, 2013).
Adansonia, the baobab
tree, an iconic genus that is endemic to the Kimberley region of
Australia, Africa and Madagascar, is a striking example of this.
These giant trees that are long lived, hold significant cultural
symbolism, and as a food source that had multipurpose value, and also as
medicine, water storage, shelter and raw material for artisanal [?
artistic] purposes in all the places they are endemic to (Adanson, 1771;
De Caluwé, Halamová & Van Damme, 2009; De Caluwé, Halamová, Van Damme,
2009; Gebauer, El-Siddig & Ebert, 2002; Livingstone, 1861; Patrut et
al., 2013; Sidibe & Williams,
2002; Swart, 1993; Wickens, 1982; Wickens & Lowe, 2008). The
distribution of the African baobab species,
Adansonia digitata, L.,
has been linked closely to dispersals of humans and settlement patterns
(Armstrong ed., 1979; Duvall, 2007), though there is no evidence of them
being cultivated historically. This association is also recognised by
the diversity and borrowing of terms for baobabs between language groups
in Africa (Blench, 2007; Wickens, 1982). Previous research on the
evolution and geographic distribution of the Australian baobab,
Adansonia gregorii, F.
Muell, in contrast, has been based on the assumption of long-term
natural processes (Baum, 1995), with no significant influence by human
agency. Rangan suggests the assumption may have been based on the view
that is long-held of Aboriginal Australia as a ‘continent of
hunter-gatherers’ (Barker, 2006; Bean, 2007; Bellwood, 2005; Denham,
Donohue & Booth, 2009; Lourandos, 1997) where anthropogenic agency was
limited to ‘firestick farming’ of landscapes for nomadic foraging and
hunting (Bleige-Bird, Bird, Codding & Jones, 2008; Jones, 1969). In this
study Rangan explored the role in humans in shaping the evolution of
A. gregorii by the
determination of whether the geographic distribution of genetic
diversity, explained partially by patterns of human migration, as was
inferred by linguistic analysis.
It is shown by levels of genetic diversity that
A. gregorii separated
from other species of
Adansonia more recently
than the breakup of Gondwana, though prior to the arrival in Australia
of humans (Baum, 1995; Pettigrew et
al., 2012). It was
demonstrated (Leong-Pock Tsy et
al. (Leong Pock Tsy et al.,
2009) that seeds of
A. digitata retain their
viability in seawater, which makes dispersal by ocean currents feasible.
It can be inferred from this that
A. gregorii arrived in
Australia before humans. It is also possible that the species arrived in
Australia more recently from a population that is not known and is now
extinct, though this is less parsimonious. One hypothesis that outlined
about how
A. gregorii may have
arrived in Australia with humans has been explored in more detail
Pettigrew (Pettigrew, 2011). Known as ‘boab’ in Australia,
Adansonia gregorii is
mainly distributed across the Kimberley region in northwestern
Australia, and there are small extensions eastwards into the Victoria
River District in the Northern Territory. The
A. gregorii distribution
in the Kimberley extends from the northern coastline the edge of the
Great Sandy Desert and the Tanami Desert (Wickens & Lowe, 2008;
Pettigrew, 2011; Baum & Handasyde, 1990; Brock, 1988; Gillison, 1983;
Mueller, 1893). The westernmost extent of the Australian Monsoon Tropics
(AMT) is represented by the Kimberley region. In urban centres of
northern Australia this tree has been introduced more recently for
ornamental purposes (Wickens & Lowe, 2008).
To the south the AMT is bounded by arid habitats, which began to develop
in the Late Cainozoic and contain biota that is distinctly different
(Bowman et al., 2010; Byrne
et al., 2008). In
northwestern Australia the major biogeographical divide is between the
Kimberley to the west and Arnhem Land to the east, with barriers that
are more localised and specific that were formed by major river drainage
systems (Eldridge, Potter & Cooper, 2011; Hill & Johnson, 1995; Oliver,
Adams & Doughty, 2010; Potter et
al., 2012). Phylogeographic patterns for rock wallabies (Petrogale
spp) and other species within the Kimberley suggest an East-West divide
that runs through the central Kimberley (Bowman et
al., 2010; Hill & Johnson,
1995; Potter et al., 2012). A
previous detailed population genetic analysis of
(A.
gregorii) has demonstrated that there is little genetic
structure, with Fst
values that are non-significant between most populations (Bell et
al., 2014), in spite of
evidence of biogeographic barriers. An arrival in the Kimberley that is
relatively recent, a recent genetic bottleneck, or high dispersal rates
across the range of the species, could explain the low geographic
structure. High dispersal is the most likely explanation, for reasons
that are detailed in Bell et al.,
(Bell et al., 2014).
In this paper the aim was to evaluate the latter hypothesis – that the
low levels of genetic structure within
A. gregorii result from
high levels of gene flow and, specifically, the dispersal of seeds being
mediated by humans has been an important evolutionary factor in the
history of the species. Occasionally,
A. gregorii may be
pollinated by birds or bats, though it is the hawkmoth that is the major
pollination agent (Wickens & Lowe, 2008; Baum & Handasyde, 1990; Bowman
1997; Baum, 1995; Lowe, 1998). Gene flow that is mediated by pollination
is limited to the paternal genome, and in species that are pollinated by
insects, it is often a mechanism of some long distance gene flow that is
less effective than fruit dispersal (Aldridge et
al., 1998; Garcia, Jordano &
Godoy, 2007; Oddou-Muratoria et
al., 2001). Some dispersal by fruit (Baum, 1995; Wickens, 1982)
could be explained by floodwaters, though this mechanism would probably
not spread seeds beyond the edges of seasonal waterways and alluvial
flats because of the fragile and dehiscent nature of the pericarp of
A. gregorii (Wickens &
Lowe, 2008; Bowman, 1997).
Mammals such as rock wallabies (Petrogale
spp.), other wallabies and kangaroos (Macropus
spp.), which eat the fruit and disperse the seeds in their scat (Wickens
& Lowe, 2008), are other possible seed dispersal mechanisms.
Phylogeographic studies of the short-eared rock wallaby (P.
brachyotis) have shown, however, that there was strong genetic
structure, which suggests that at least this species has limited ability
to disperse seeds across environmental barriers in the Kimberley
(Potter, Eldridge, Taggart & Cooper, 2012; Potter et
al., 2012).
Anthropogenic agency has not been formally considered as a vector of
gene flow for
A. gregorii, in spite of
archaeological evidence of its long-term use by Aboriginal groups in the
region (Pettigrew, 2011; Boland & Brooker, 1985; McConnell & O’Connor,
1997; McConnell & O’Conner, 1999; Wallis, 2001). This omission may have
resulted from the fact that the boab was not cultivated historically, or
not considered to be part of the food crops that were culturally
harvested by Aboriginal people. Also, the presence of the remains of
boab fruit at 1 or 2 archaeological sites is not enough to consider
dispersal by humans across the geographic range of the species. Evidence
is also required to evaluate whether humans have played a role in gene
flow of the boab. According to Rangan they reasoned that historical
linguistics, which has been used combined with data sources to trace
geographic diffusion patterns of domesticated species (Donohue & Denham,
2009; Perrier et al.,
2011; Roullier, Benoit, McKey & Lebot, 2013), could be applied to
a tree species that was not domesticated such as
A. gregorii (Bostoen,
Grollemund & Muluwa, 2013). Specifically, in this study, they
investigated the role human agency played in gene flow of
A. gregorii by testing
for congruence between the spatial distribution of genetic variation
in
A. gregorii trees and
associated word forms of Aboriginal languages of northwest Australia.
They demonstrated that there was a high degree of spatial overlap
between the genetic and linguistic data. It is indicated by the results
of this study that, as has been shown previously for
A. digitata in Africa
(Duvall, 2007; Assogbadjo et al.,
2010; Leong Pock Tse et al.,
2009), the genetic diversity of
A. gregorii in northern
Australia has been influenced by ancient humans, probably by acting as
agents of seed dispersal over long distances.
The hypothesis that dispersal by humans has played a role in shaping the
geographical distribution of
A. gregorii is supported
by the results of this study. In northwestern Australia the limited
morphological and genetic divergence of the genus
Adansonia can be
attributed to gene flow within
A. gregorii, in spite of
the presence of biogeographic barriers. The lack of evidence of barriers
to gene flow is explained most easily by a long history of humans moving
boab seeds, given the lack of other seed dispersal agents. The
hypothesis of gene flow by human-mediated dispersal of
A. gregorii is
supported further by the concordance between gene flow and
loanword diffusion. An indication of patterns of interaction between
nPNy language families
and
PNy subgroups that would
have influenced the distribution of
A. gregorii in the
Kimberley region is provided by the movements of loanwords across all
boab populations. At Carpenter’s Gap Shelter in the Napier Ranges of the
Kimberley, which is within the Banuba language area (McConnell &
O’Connor, 1997; McConnell & O’Connor, 1999), Additional evidence of
anthropogenic agency is facilitated by gene flow. Continuity of human
occupation that extended over 40 ka into the Late Pleistocene was shown
by the presence of lithic and macrobotanical remains. Fragments of boab
pods recorded dates of 39 ka, 20 ka and 18 ka and 15 ka (McConnell &
O’Connor, 1997; McConnell & O’Connor, 1999) and from 3 ka onwards,
peaking at 650 years ago, a substantial increase in deposition of pods
(McConnell & O’Connor, 1999). Further evidence of long-term consumption
by humans of the fruit (McConnell & O’Connor, 1997), was provided by the
presence of old boab trees near Aboriginal middens in western Kimberley.
In the northern Kimberley prehistoric rock art also shows depictions of
the tree, which indicates its cultural significance to ancient groups
that may have occupied this region (Pettigrew, 2011).
It was postulated by Rangan that, based on the above evidence, that
recent evolution and geographic distribution have been shaped primarily
by ancient human agency. It is implied by the phylogenetic tree of boab
populations and the predominant direction of gene flow together, that
the source populations of
A. gregorii gene flow
were most likely in the extreme northwestern Kimberley, which
potentially overlaps with the inferred NC population area. According to
Ragan it is likely that the range of this source population extended
beyond the coastline of the present at the Last Glacial Maximum (LGM,
roughly 20 ka) when the sea levels were more 120 m below what it is at
present, and the northwest continental shelf was exposed to the maximum
extent (Kershaw, 1995). The increased exposure of land surfaces of both
the Sahul and Sunderland shelves, lower sea surface temperatures (SST),
and oceanic currents that were altered as the result of the closure of
several shallow seas and passageways between these continental shelves
contributed to a northwards shift of the Inter-Tropical Convergence Zone
(ITCZ), and thereby reducing seasonal precipitation levels (Decker,
Tapper & van der Kaars, 2002), resulting in the formation of semiarid
savannah conditions (Bird, Taylor & Hunt, 2005) in which ancestral
populations of boab would have existed in northwest Australia.
Subtropical desert conditions would have prevailed across much of the
exposed continental shelf beyond the Kimberley of the present (Reeves et
al., 2013; Hesse, Magee & van
der Kaars, 2004; Fitzsimmons et
al., 2013; Wyrwoll & Miller, 2001), with rainfall as much as 30-50%
below those of the present (De Dekker, Tapper & van der Kaars, 2002) and
higher levels of aeolian landform activity (Bowler, Wyrwoll & Lou,
2001). The distribution of
A. gregorii would have
been limited by these arid conditions with low levels of rainfall in
particular, as the current distribution of the species coincides with
areas that receive at least 700 mm of annual seasonal rainfall (Beard,
1967). It is likely, therefore, that under the drier climatic conditions
during the LGM that resulted from the northwards shift of the ITCZ (De
Dekker, Tapper & van der Kaars, 2002), that populations of boab would
have been limited to the extreme northern coast of the Kimberley of the
present, as well as to the continental shelf.
During the postglacial period and the Pleistocene-Holocene transition,
the subsequent sea level rise and restoration of monsoonal activity
between 17 and 6 ka the Sahul Shelf was flooded and established the
coastlines of northern Australia of the present (Kershaw, 1995; Hesse,
Magee & van der Kaars, 2004). The flooding of the continental shelf
beyond the Kimberley coast of the present, as well as increased
monsoonal rainfall over inland Kimberley, would have altered the
distribution of ancestral populations of
A. gregorii and possibly
led to a genetic bottleneck from which the current populations would
have expanded (Bell et al.,
2014). According to Rangan it was proposed that ancient human groups
living on the coast along the shelf that had previously been exposed
during the Late Pleistocene would have retreated from the inundated
areas, carrying boab fruit with them as they migrated further south and
east.
Records from the Pleistocene of boab remnants at the Carpenter’s Gap
archaeological site (McConnell & O’Connor; McConnell & O’Connor, 1999)
may represent sporadic visits of human settlers from the north
(McConnell, 1997). The site records the presence of shells and beads
from the Early Holocene, which suggests the movement of high value goods
from the coast (O’Connor et al.,
2014). Evidence of occupation at Carpenter’s Gap, in common with other
archaeological sites in Australia, increases sharply from the Middle to
Late Holocene, which
possibly reflects a demographic expansion in southern Kimberley
(Williams, 2013).This
population increase, along with other factors such as climate and
vegetation change (McConnell & O’Connor, 1999; Wallis, 2001), growth in
the local populations of boab, and the more frequent use of the site for
cultural ceremonies and exchanges (McConnell, 1997), could explain the
increase in the remnants of boab pods, as well as other foods and seeds
from about 3 ka to 650 years ago (McConnell & O’Connor, 1997).
It is likely that in the Late Holocene the increased mobility of
Aboriginal groups in the southern Kimberley could have been influenced
by greater climatic variability in northern Australia. The Late Holocene
(~1,000 BCE – 500 CE) the climate in the Australian Monsoon tropics was
marked by periods of increased seasonality and aridity (Lees & Clements,
1987; Brockwell et al., 2013;
Shulmeister, 1999). Rangan suggests these conditions may have
contributed to increased mobility of Aboriginal groups between different
parts of the Kimberley and would have contributed to higher gene flow of
A. gregorii by the
dispersal of fruit seeds, which would have been accompanied by diffusion
of word forms.
Additional indication of patterns of migration and social interaction
that could have contributed to the spread of
A. gregorii is provided
by linguistic data. During the Late Holocene the periods of aridity may
have affected the survival of desert-based Marngu (PNy subgroup)
speakers, leading to the migration into southern and western Kimberley.
Rangan suggests the word form
larrkarti was probably coined by Marrngu speakers during this period
when they would have encountered the boab tree in the landscape of the
Kimberley. The mobility and cultural interactions with the mobility and
cultural interactions of the PNy groups with neighbouring nPNy language
groups such as the Nyikina and the Banuba groups would have increased at
the same time. Evidence that the loanword
larrkarti is of a relatively
recent nature is demonstrated by the way in which it has been
incorporated into other languages, generally in a form that has been
unchanged (McConvell & Laughren, 2004). The Gija language (Southern
Jarragan has retained its inherited word
jumulu for the tree and
adopted the larrkarti for the
fruit pod, possibly indicating its salience and the portability of the
edible seed pod in the more recent borrowing. Likewise the Jaru language
(PNy Ngumpin subgroup) has no inherited words for boab and uses both
Jamula (modified from Gija)
and larrkarti for the tree. Other examples of recent expansion of the
boab and loanword diffusion further east is present in the Ngarinyman
language (Ngumpin) in the Northern Territory, where the words jang-nge
(borrowed from miriwong, meaning for eating) and
jumulu (borrowed from Gija)
are used for the fruit or its edible pith, and the tree respectively. In
the Kimberley, the possible climatic influence on human migration and
movement of the boab loanword echoes some aspects of Bostoen et
al. (Bostoen, Grollemund &
Muluwa, 2013) of climate-induced dynamics and expansion of Bantu in
Africa. The PNy Marrngu term larrkarti, as well as other pre-existing
words, such as wajarr, jumulu,
and kertewun moved across the
Kimberley in patterns that corresponded with multidirectional boab gene
flow as was shown in this study, though there is no analogous evidence
of a large-scale expansion of a single linguistic group into the
Kimberley. Rangan suggests the high gene flow in
A. gregorii appears to be
similar to the case of
A. digitata in Africa,
where human agency has been involved in dispersal of the species
(Duvall, 2007; Assogbadjo, 2010; Leong Pock Tsy, 2009). The boab
loanword movement in the Kimberley may be compared with Blench’s
(Blench, 2007) account of the spread of Bantu words for
A. digitata. He noted
that the genetic diversity of baobabs in the ecological zones of West
Africa and the diversity of vernacular names for the African tree
suggests considerable antiquity as well as well as significant east-west
movement along trade routes and exchanges of associated terms and
terminology. In spite of the diversity of baobab names, however, he
points out that 2 competing Bantu roots,
#mbuyu and
#muramba and variations of
these, are present in the Bantu languages of southern and eastern
Africa. According to Rangan it is said that the Bantu expansion from the
tropical forests of West Africa to have begun from about the middle of
the first millennium BC (~2,500 BP onwards) and by 500 CE reached
southern Africa (Bostoen, Grollemund & Muluwa, 2013; Bostoen, Grollemund
& Muluwa, 2013; Ehret, 2001). It is argued by Blench that the Bantu
would not have been familiar with the baobab as it doesn’t grow in areas
of tropical forest in Cameroon, Gabon and Congo where the protolanguage
is believed to have originated. The tree would have been encountered by
them as they expanded eastwards and emerged into the savannah, where
they developed new terms by borrowing from resident hunter gatherer
groups or comparing it with some species of trees they already knew. In
the Bantu languages if eastern and southern Africa the loan or
variations of baobab words mbuyu
and muramba would therefore
indicate the movement of Bantu into these Areas (Blench, 2007). The
expansion of Bantu over this 3,000 year period and their contribution to
high levels of gene flow in baobabs across these regions may have
resulted in the lower levels of diversity of
A. digitata in eastern
and southern Africa that was detected by Leong Pock Tsy et
al. (Leong Pock Tsy, 2009).
Conclusion
The limited intraspecific divergence within
A. gregorii in Australia
is probably the result of high gene flow that was mediated by human
agency, similar to that inferred by
A. digitata in
continental Africa (Leong Pock Tsy, 2009; Assogbadjo, Gele Kakai, Kyndt
& Sinsin, 2010), combined with suitable habitat shifts and a weak
bottleneck following the end of the LGM (Bell et
al., 2014). The use of
Adansonia by humans over
many thousands of years on both continents would have contributed to
gene flow over long distances and across biogeographical barriers. It
could be, in contrast, that the divergence of
Adansonia into 6 species
in Madagascar was possible partially because humans were not present
until about 2 ka. This hypothesis can only be tested by investigation of
the ecological, physiological and biogeographical processes that
contributed to the speciation within the
Adansonia clade from
Madagascar.
According to Rangan et al.
this study has contributed new evidence to the role of humans in
influencing the evolution and distribution in Australia of a
non-domesticated plant species. For a long time Australia has been
viewed as a continent of hunter gatherers (Denham, Fullagar & Head,
2009), where prehistoric and pre-contact Aboriginal populations played
minimal roles in selecting and dispersing useful plants (Denham, Donohue
& Booth, 2009; Lourandos, 1997). This assumption has been challenged by
some recent studies, however, by providing evidence that ancient
Aboriginal groups had a role in the dispersal of food plants across the
continent (McConnell, 1997; Kondo et
al., 2012; Gott, 1983).
Included among these are bananas, (Musa
spp.) taro (Colocasia
esculenta), in northern Australia (Denham, Donohue & Booth,
2009),
Livistona (Kondo et
al., 2012) palms in Central
Australia and some yams (dioscoria
spp.) in northern Australia (Kondo et
al., 2012), and yam daisy (Microseris
scapigera) in southeastern Australia (Gott, 1983). New insights
regarding the influence of human agency in the evolution and
distribution of the boab, an important non-cultivated food plant species
that shaped the long-term landscape and environmental history of
northwest Australia, by the findings of this study.
Rangan, H. (2015). "New genetic and linguistic analyses show ancient
human influence on baobab evolution and distribution in Australia."
PLoS ONE 10.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |