Actualité scientifique
N° 379
July 2011
The tropical rainforests
(TRFs) harbour a multitude
of living resources. Where,
when and how did this
wealth of biodiversity
appear? To find answers to
these fundamental
questions, IRD biologists
and their research
partners1 have realized the
first evolutionary history of
a family of plants
characteristic of this
ecosystem: palms2
(Arecaceae or Palmae).
Using a molecular dating
method, based on DNA
sequences, they traced
this history back to the
middle of the Cretaceous3,
the final epoch of the
dinosaurs. The palms were
found to start diversifying
in Laurasia –the ancient
supercontinental landmass
in the Northern
Hemisphere– around 100
million years B.P., not at
the Equator only 65 million
years ago as previously
assumed.
Applying the molecular
clock hypothesis4, the
research team then
determined the speciation
rate. The TRFs’ species
richness is the result of
constant diversification
probably due to the
continuing existence of
this ecosystem since its
formation. At the time of
glaciations and massive
extinctions they were
refuge zones, but now they
are under serious threat
from human activities.
Actualidad cientifica
Tropical rainforests
original generators of biodiversity
© IRD / T. Couvreur
Scientific news
The palms have been an important constituent part of the TRFs (here in Brazil and French Guiana) since their formation 100 million years ago.
Tropical rainforests (TRFs) together are the Earth’s
richest terrestrial ecosystem, with 50% of all
known species of plants and animals. They represent one-third of the globe’s forest formations, now
clothing the continents of Africa and Latin America
and also the Indo-Pacific Basin. Determination of
where and when they were formed and how their
rich biodiversity developed remains a crucial issue
for biologists.
Forests 100 million year old
An IRD team and its research partners1 recently
showed that these forests began to form more than
100 million years B.P. (before present). Another
theory had postulated that they only appeared 35 to
45 million years later –not long after the extinction of
the dinosaurs at the beginning of the Tertiary era.
Their work resulted in the first complete genus-level
evolutionary history of one of the most characte-
FOR FURTHER INFORMATION
ristic plant families of the TRFs: the palms (Arecaceae or Palmae). Using a molecular dating method
–i.e. based on DNA sequences calibrated with
fossils– they constructed a dated phylogeny tracing
the origin of existing palm lineages at 100 million
years B.P., the middle Cretaceous3. The forests were
therefore already forming at this time.
2
Forest evolution, slow but sure
With the age of the palms now established, the
molecular clock hypothesis4 was applied to estimate
the dates when the different species appeared. The
conclusion was that the major palm lineages
emerged gradually over geologic time. This finding
enabled the research team to demonstrate that the
build-up of TRF biodiversity was a process of
constant diversification in an ecosystem that had
persisted right from its origin. Opposite to the case
of the temperate forests, for example, which largely
disappeared during the last Ice Ages. Thus the principal lineages lived on, constantly accumulating and
diversifying, and generated the biodiversity we
observe today.
This result fits in with a previous study on another
family of tropical flowering plants, the Annonaceae,
but it contrasts with the hitherto generally accepted
theories that this diversity stemmed from rapid
speciations which occurred in distant times followed
by ecosystem saturation that caused them to slow
down. If more recent, they would stem from an
acceleration of the diversification rate stimulated by
mountain building, such as the uplift of the Andes,
leading to greater spatial heterogeneity and therefore high speciation activity.
history and the geographical position of the ancient
continents. This information enabled them to determine the area of origin of palms and hence of the
present tropical forests. In fact these forests arose in
the Northern Hemisphere, on Laurasia –the supercontinent made up of what is now North America
and Eurasia– not in the Equatorial latitudes. The
equatorial climate of the time, the Cretaceous, was
too hot and dry for humid forests to survive.
Contact
Thomas COUVREUR,
researcher at the IRD
Tel: +33 (0)4 67 41 63 09
thomas.couvreur@ird.fr
UMR232 Diversité, Adaptation,
Développement des plantes - DIADE
(IRD/Université Montpellier 2)
Address
IRD
911 avenue Agropolis
BP 64501
34394 Montpellier cedex 5
A rich diversity little explored
The history of TRFs remains poorly documented for
this geological era owing to gaps in the fossil record
related to climatic conditions unfavourable to fossilization5 and although it was known that the TRFs
were relatively old, their exact age, geographical
location and mechanisms of diversification were still
poorly understood.
Unlike the other species of this ecosystem, the
palms have left quite substantial fossil records, as
their characteristic morphology makes them easily
recognizable. The biologists already had a good
knowledge of the taxonomy of this family, down to
genus and species level. Applying this they have
reconstituted the family’s phylogeny, showing its
evolutionary lineages and interrelationships.
References
Couvreur Thomas L.P., Forest F.,
Baker W.J. Origin and global diversification
patterns of tropical rain forests: inferences
from a complete genus-level phylogeny of
palms. BMC Biology 2011, 9:44.
doi:10.1186/1741-7007-9-44
Couvreur Thomas L.P., Pirie M.D.,
Chatrou L.W., Saunders R.M.K.,
Su Y.C.F., Richardson J.E., Erkens R.H.J.
Early evolutionary history of the
flowering plant family Annonaceae:
steady diversification and boreotropical
geodispersal. Journal of Biogeographie
2011, 38(4):664-680.
doi: 10.1111/j.1365-2699.2010.02434.x
The TRFs have acted as refuge zones for the
palms over the millions of years of their evolutionary history. Today their extent represents 7% of
the Earth’s surface. They are hotpots of biodiversity, rich with a host of resources (wood, fruit,
game, land reserves) and provide the livelihoods of
several hundred million people. But because of this
great wealth of resources they are now seriously
under threat.
Key words
Tropical rainforests, palms, evolution
On the same subject:
Des forêts et des hommes
Forest origins in the North
The rainforests have not always grown along the
Equator. The research team looked into the prevailing climate of 100 million years ago, the tectonic
« Les forêts tropicales
humides, avenir de la
planète »
Copy editor – Gaëlle Courcoux - DIC, IRD
Translation – Nicholas FLAY
1. This work was conducted in partnership with the New York Botanical Garden in the USA and the Royal Botanic Garden in the UK.
Coordination
Gaëlle Courcoux
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2. Although a few of the 2.500 palm species provide the greenery that shades oases or sea shores, 90% of them are firmly restricted
to the TRFs.
3. The Cretaceous period (145-65 million years B.P.) was the time of the last mass extinctions our Earth has known, when the dinosaurs
were wiped out. However, it also gave rise to the diversification of flowering plants, making it vitally important in the evolutionary
history of plants.
4. The molecular clock hypothesis considers that the mutation rate of genes is correlated with the evolution rate of species.
TRFs harbour a multitude of resources. The palms, for example, give highly nourishing fruit and provide materials (leaves, fibres etc.) for house construction or
manufacturing a range of objects (rope etc.).
© IRD / L. Emperaire
© IRD / P. Dumas
© IRD / J-C Pintaud
© IRD / MN Favier
5. Heat and humidity together quickly decompose organic matter.
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