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.
Tropical rainforests together are 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 partners 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 characteristic 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 Cretaceous. The forests were therefore already forming at this time.
Forest evolution, slow but sure
With the age of the palms now established, the molecular clock hypothesiswas 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.
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 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.
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 fossilization.5 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.
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 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.
1. This work was conducted in partnership with the New York Botanical Garden in the USA and the Royal Botanic Garden in the UK.
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.
5. Heat and humidity together quickly decompose organic matter.
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