Origin and Significance of Tropical Rainforests

Growing up in coastal British Columbia, I had access to rainforests. I loved the intense green colors, the roaring rivers and waterfalls, the shade, the wildlife, and the abundant food. The beauty was so stunning that I didn’t mind the intense rain, fog, and darkness.

It is easy, though, to take rainforests for granted. Thanks to a recent research study, we now know that rainforests, especially angiosperm-dominated stratified tropical rainforests, are a relatively recent phenomenon.

There are two types of seed-bearing plants: gymnosperms and angiosperms. Gymnosperms are nonflowering plants and their seeds are bare. Angiosperms are flowering plants and their seeds are enclosed within an ovary. Gymnosperms are evergreen and have needle- or scale-like leaves. Angiosperms have a seasonal life cycle and have broad, flat leaves.

The first rainforests were lycopsid rainforests comprised of giant ferns. They first appeared 390 million years ago during the early part of the Late Devonian period.¹ Gymnosperm rainforests replaced the lycopsids beginning about 320 million years ago during the Late Carboniferous period. While angiosperms date back to about 135 million years ago,² rainforests that are predominantly angiosperms did not appear until about 60 million years ago. In an attempt to determine how and when angiosperm rainforests originated and how their origin made possible human civilization, a team of 21 paleontologists and ecologists, led by Mónica Carvalho and Carlos Jaramillo, conducted a decade-long study in the rainforests of Colombia.³

Origin of Angiosperm Rainforests
The Carvalho-Jaramillo team collected more than 50,000 grains of fossilized pollen and more than 6,000 leaf fossils from 39 different sites in Colombia that spanned the date range from a million years before the end-Cretaceous mass extinction event to 6 million years after the event. A 10-kilometer diameter asteroid struck Chicxulub in Mexico’s Yucatan Peninsula 66.038 +/- 0.049 million years ago, wiping out the dinosaurs and resulting in the extinction of three-quarters of all plant and animal species on Earth.⁴ This impactor brought about the second greatest known mass extinction event, second only to the Permian-Triassic mass extinction event 251 million years ago.

The research by the Carvalho-Jaramillo team revealed two very different tropical forests on either side of the end-Cretaceous mass extinction event. The leaf and pollen fossils that date before the impactor striking the Yucatan Peninsula show only a narrow range of carbon-12 to carbon-13 isotope ratios. However, the fossils that date a few million years after the impact event manifest a wide range of carbon-12 to carbon-13 isotope ratios. The leaf fossils dating before the impact event reveal a small range of leaf vein density while leaf fossils dating after the impact display a large range of leaf vein density. When Carvalho’s team looked for signs of leaf damage by insects, they found a consistent pattern for leaf fossils dating before the impact event and an inconsistent pattern for leaf fossils postdating the impact event. 

The only possible explanation for what the Carvalho-Jaramillo team observed is that the Colombian forests before the impact event were open canopy, single-layered forests. After the impact event, they were closed canopy, multi-layered forests. Large ranges of leaf density venation, carbon-12 to carbon-13 isotope ratios, and insect leaf damage are all indicative of forests with dense, stratified canopies. In such forests, leaves at the top of the canopy are designed to take advantage of bright, near continual sunlight and leaves at the bottom of the canopy are designed to optimally function in dark, near continual shade.

The study by the Carvalho-Jaramillo team determined that Colombian forests before the impact event were dominated by gymnosperm species. After the impact event, angiosperms dominated and accounted for 90 percent of the plant species.

Cause of the Transition
Carvalho, Jaramillo, and their colleagues proposed three possible explanations for the transition from tropical forests dominated by gymnosperms to those dominated by angiosperms. The first was that after the impact event there were no large dinosaurs to trample down the space between large conifer trees and to eat the saplings. The second was a change in soil nutrients. The asteroid impact would have dramatically increased the availability and diversity of nutrients in the soil. The third is that large conifer trees are more vulnerable to dying out than large deciduous trees. The post-impact environmental conditions may have strongly favored the survivability of angiosperm trees compared to gymnosperm trees.

I can suggest a fourth possible explanation. Heavier precipitation patterns after the impact event would have favored angiosperm trees over gymnosperm trees.

The Carvalho-Jaramillo team strongly argued for a combination of explanations. They showed how nothing less than the operation of all three of their explanations could adequately explain the occurrence of the dramatic transition.

Implications of the Transition
In their paper, the Carvalho-Jaramillo team did not address the philosophical or theological implications of their discovery. An obvious implication is that the development of angiosperm-dominated closed-canopy tropical rainforests greatly expanded the diversity of Earth’s species. At least 40 percent and as much as 75 percent of all biotic species exist in tropical rainforests. It is thanks largely to tropical rainforests that Earth, for the first time in its history, possesses close to the theoretical maximum number of species. Thanks to this enormous diversity of species, we humans have been able to harvest chemicals, drugs, and bioinspired designs to launch and sustain our global high-technology civilization.

The Amazon rainforest alone accounts for 20 percent of the oxygen we breathe and a near equal percentage of removal of greenhouse gases from the atmosphere. Much of the credit for the past 9,500 years of extreme climate stability goes to the world’s tropical rainforests. If it were not for the widespread appearance of tropical rainforests 60 million years ago and their maintenance throughout the past 60 million years, the increasing luminosity of the Sun would not have been adequately compensated. Our planet would have become completely sterile and our existence would have been rendered impossible. The just-right timing, extent, and diversity of angiosperm-dominated rainforests testify of a Mind who was intent on preparing a home for human beings.

Endnotes

1. Christopher M. Berry and John E. A. Marshall, “Lycopsid Forests in the Early Late Devonian Paleoequatorial Zone of Svalbard,” Geology 43, no. 12 (December 2015): 1043–1046, doi:10.1130/G37000.1.
2. Mario Coiro, James A. Doyle, and Jason Hilton, “How Deep Is the Conflict between Molecular and Fossil Evidence on the Age of Angiosperms?” New Phytologist 223, no. 1 (July 2019): 83–99, doi:10.1111/nph.15708; Patrick S. Herendeen et al., “Palaeobotanical Redux: Revisiting the Age of the Angiosperms,” Nature Plants 3 (March 3, 2017): id. 17015, doi:10.1038/nplants.2017.15.
3. Mónica R. Carvalho et al., “Extinction at the End-Cretaceous and the Origin of Modern Neotropical Rainforests,” Science 372 (April 2, 2021): 63–68, doi:10.1126/science.abf1969.
4. Paul R. Renne et al., “State Shift in Deccan Volcanism at the Cretaceous-Paleogene Boundary, Possibly Induced by Impact,” Science 350, no. 6256 (October 2, 2015): 76–78, doi:10.1126/science.aac7549; Mark A. Richards et al., “Triggering of the Largest Deccan Eruptions by the Chicxulub Impact,” Geological Society of America Bulletin 127, nos. 11–12 (November 2015): 1507–1520, doi:10.1130/B31167.1; Hugh Ross, “Dinosaur Extinction Challenges Evolutionary Paradigm,” Today’s New Reason to Believe (December 10, 2015).