It is into this fiery Cretaceous (143.1 million to 66.0 million years ago) breakup that the angiosperms entered the frame. After a cooler interlude in the Triassic that favored conifer expansion, this world-making process released masses of CO2 into the atmosphere as a product of volcanic eruptions, stimulated by the grinding apart of the Earth’s crust. Temperatures and humidity rose, creating the ideal “greenhouse” for the first angiosperms that were able to ride their way into a new world order on the backs of free-roaming continents. Based on the majority of existing genetic and fossil evidence, these flowering plants likely first emerged around 140 million to 120 million years ago in the tropical latitudes, although some researchers have also suggested that midlatitudes would have perhaps provided more suitable, stable settings for angiosperm emergence during the Early Cretaceous (143.1 million to 100.5 million years ago) and others still argue that the angiosperms may have begun to emerge as early as the preceding Jurassic. Regardless, analysis of ancient angiosperm lineages certainly makes clear that they first appeared as part of a warm, wet forest undergrowth, lying low under gymnosperm canopies that were gradually losing diversity. Meanwhile, genetic evidence shows that around 100 million years ago, many orders of angiosperm widespread in tropical forests today diverged as Gondwana broke up and global temperatures became warmer and wetter. Between 100 million and 70 million years ago, angiosperms spread across the tropics, experiencing rising species diversity and a growing domination over all other plant groups, particularly in warm, wet land-based environments. One final dramatic proliferation approximately 60 million years ago saw angiosperms spread extensively across the majority of the lower and middle latitudes. This proliferation of angiosperms has also been implicated in alterations to the atmosphere and the water cycle. They have higher numbers of veins in their leaves and can therefore perform water evaporation (transpiration) and photosynthesis more effectively. The release of more water back into the atmosphere from these novel leafy surfaces has been linked to the development of wetter and less seasonal climates.7
The scale of the tectonic journeys of the angiosperms is visible when we focus in on the family that includes some of the true tropical rainforest giants that stand today (some as high as eighty to one hundred meters), including major tropical timber trees that are exploited globally, the Dipterocarpaceae. Evidence from fossils, modern genetics, and comparisons of body shapes all indicate an origin for this group before 120 million years ago on the landmass of Gondwana, which included Africa, South America, Antarctica, Australia, and the Indian subcontinent. Some members of this family then rafted on top of the plate comprising India and Sri Lanka as it separated from Gondwana and eventually smashed into Laurasia (a plate including the rest of Asia), completing their global voyage about 45 million years ago. Aided by the forces of geology and perhaps also pollinating insects, the angiosperms accompanied global “greenhouse” conditions that prevailed from the Cretaceous (143.1 million to 66.0 million years ago) right up until the middle Eocene (48.1 million to 37.7 million years ago). As early as the Late Cretaceous (100.5 million to 66.0 million years ago), these flowering plants had also already started to become key members of tropical forest ecosystems, and fossils of broad, woody angiosperm trees and fruit-producing climbers have been found in Nigeria. However, clear fossil records of the earliest angiosperm-dominated forest ecosystems, with a collection of species and a structure (i.e., a number of stratified layers, or “stories,” within closed forests, where light reaching the floor is limited) that we expect from tropical forests today, only really began to appear in the Paleocene (66.0 million to 56.0 million years ago), when temperatures started to rise and environments began to recover following the extraterrestrial collision and volcanic winter that drew a dramatic line under the Cretaceous and dinosaurian dominance. To see what these new angiosperm-dominated tropical forests looked like and how they related to the diverse ecosystems we know from the tropics today, we must now follow Carlos and his team into yet another mine in the Americas.8
THE CERREJÓN COALMINE in northern Colombia is one of the largest open-pit coalmines in the world. So vast is the expanse of coal in the region that it has been recognized as its own geological entity, the Cerrejón Formation. As we have already seen in Chapter 1, thick beds of coal are something of a “smoking gun” for detectives in search of ancient tropical forests, and these are no exception. The mine has opened an unprecedented window into the communities of plants that formed in the tropics 60 million to 58 million years ago, just after the dramatic Cretaceous-Paleogene boundary (66.0 million years ago) extinction event that wiped nonavian dinosaurs off the face of the planet. The Cerrejón coalmine represents a treasure trove not just for fossil fuel companies but also for paleobotanists, and to date it has produced over 2,000 plant “mega fossils,” some nearly twice as big as an average human, which include ancient leaves, flowers, seeds, and fruits. Today, when we think of tropical forests, particularly rainforests, in the Neotropics (i.e., the tropical portions of the Americas), we imagine—or, in the case of Victor and myself, experience—a certain level of plant diversity, the presence of specific angiosperm families, and high numbers of plant-eating insects, high temperatures, and plentiful rainfall. Unsurprisingly, these traits are immensely difficult to find altogether in one spot in the fossil record, making it exceedingly challenging for us to properly identify the origins of this evocative twenty-first-century tropical forest community. The Cerrejón coalmine is perhaps the major exception in this regard and represents a paradise for researchers like Carlos.
Carlos and his team have discovered that the plants found at Cerrejón represent many of the families that dominate tropical rainforests in this part of the world today. When analyzing the fossils under the microscope, the team could identify characteristic plants, including palms, legumes, and even the ancient relatives of the avocado, and show that angiosperms had truly come to dominate these new, post-dinosaurian forests, with multilayered rainforest canopies providing the stage for angiosperms to diversify and experiment with the astoundingly different forms we see in contemporary tropical rainforests. Amazingly, study of the microscopic (e.g., fossil pollen) and macroscopic (e.g., fossils of leaves) remains preserved at Cerrejón can find about 60 to 80 percent of all of the main groups of plants found in twenty-first-century Neotropical rainforests. While this ancient forest was slightly less diverse, its likeness to those seen in Central and South America today is remarkable given the subsequent geological processes (e.g., uplift of the Andes between 55 million and 40 million years ago) and changes in climate (fluctuating between warmer and cooler than present). Effectively, at Cerrejón we can start to see the origins of some of the modern tropical forests we can still encounter around the world today, 50 million years before our first hominin ancestors would appear. These plant communities, while changing subtly and waxing and waning in spread, were to stand tall throughout major upheavals and remain around us, spanning a significant portion of the Americas, including, of course, the Amazon Basin. After experiments of varying degrees of success, a new type of tropical forest had now established itself on Earth, and it was here to stay.9
Figure 2.1. Image of an angiosperm leaf recovered from the Cerrejón Formation in northern Columbia by Carlos and his team. The density of veins in the leaf surface remains clearly visible after 60 million years. Carlos Jaramillo
Carlos and his team did not stop there, however. The remarkable preservation within the Cerrejón Formation even allowed them to analyze the cell and leaf structures of these ancient forests. The leaves had the same forms as those found in modern Neotropical rainforests: big with smooth edges, indicating similarly hot and wet conditions. Indeed, comparison with modern leaf shapes suggests a past rainfall greater than 2,500 millimeters per year and an average annual temperature of greater than 29°C, about 2°C higher than that of today. In fact, the leaves were so complete, even after a remarkable 60 million years, that Carlos and his colleagues could analyze the bite marks of
insects that had feasted on these tropical bounties. This damage showed that a variety of plant-eating insects now bustled around tropical forests, though, like the plants, they were less diverse than the menagerie of organisms found on the tree trunks and floors of the Amazon rainforests today. Finally, the available fossils also show the beginnings of a classic Neotropical animal community, including the remains of a now extinct giant snake, the terrifyingly named “Titanoboa.” Related to today’s boa constrictors and anacondas, this monster would have been right at home in Luis Llosa’s 1997 Amazon-based horror film Anaconda. As in the case of the large leaves, the significant size of this snake (nearly thirteen meters in length) suggests that the Cerrejón forest was warmer than the tropical rainforests of modern South America. This is because snakes are “ectothermic,” relying primarily on environmental sources of heat to regulate their body temperature.10
Cerrejón was not the only tropical forest in town. By the Paleocene-Eocene boundary (56.0 million years ago), tropical forests, with characteristic plant families and structures that we still see in these forests today, spanned the tropics. Not only that, but similar “megathermal” forests reached the greatest extent they have ever managed in the history of our planet. Although incredibly diverse and not always “tropical” in the context of the definition I provided at the start of this chapter, whether we set down in North America, Africa, South America, Australia, Asia, or Europe, warm megathermal forest ecosystems would have greeted us, reinforcing a global move toward warmer and wetter conditions as they further inserted themselves into the Earth’s systems. Mangrove-like forests even adorned the now frosty coasts of the United Kingdom, while so-called boreotropical forests (the name for forest communities with similarities to those seen in the tropics today that extended well into northern and southern latitudes) reached as far south as Tasmania and as far north as Alaska. Importantly, unlike those of the Carboniferous forests, the structure and species of the new forests in and around the equator, like that at Cerrejón, would have appeared much more familiar to those of you who live in or visit the tropics today. The distributions and evolution of various tropical angiosperm plants continued apace, and certain plant families were able to move rapidly between the tropical continents via land bridges. However, the ongoing processes of tectonic separation throughout the Eocene (56.0 million to 33.9 million years ago) meant that tropical forest communities began to take on their own regional “flavors.” By the end of this period and the final division of Antarctica, South America, and Australasia, the Earth’s continental makeup was pretty much decided. Species that had previously surfed on moving geological plates now became largely trapped. The result was the gradual, independent evolutionary formation of the unique forest ecosystems we know from different parts of the tropics today.11
The process of continental separation, as well as isolating tropical forests in different parts of the globe, also stimulated the development of new climate systems. The moving apart of landmasses opened up new spaces for vast ocean circulation systems that extended across the planet. Cold water flowed from the north polar region into the Atlantic, cooling and drying the Northern Hemisphere thanks to a reduction in evaporation, while Antarctica became separated from warmer equatorial waters. From about 47 million years ago, the formation and rise of many the world’s great mountain ranges exacerbated this trend. They began to force the megathermal forests back toward their current distribution as a belt around the Earth’s equator. While trees retreated, new players began to expand from approximately 20 million years ago: players that had crept onto the vegetation scene during the time of the dinosaurs. With a shape, root system, and physiology adapted to resisting droughts and cold snaps, grasses provided a small but collectively mighty challenge to forests during times of disturbance. And thus, present-day ecological battlelines were formed within the tropics: on one side, tropical and subtropical forest ecosystems, with their vast amounts of angiosperm diversity; on the other, novel grassland ecosystems. These two formations have been locked in an eternal skirmish ever since, waxing and waning in the face of periodic global climate swings.12
BY 10 MILLION years ago the modern tropical forest toolbox was largely complete, with characteristic plant species, continental location, and the broad global conditions we associate with them today all in place. This did not mean that they remained idle, however. Although their exact impacts remain hotly debated, changes in precipitation, temperature, and global CO2 continued to drastically impact the extent and structure of tropical forests. During the Quaternary (2.58 million years ago until present), these shifts have been driven primarily by cycles in the distribution of the Sun’s energy around the Earth, linked to our planet’s changing orbit and tilt. Broadly, over the last 1 million years these “Milankovitch” cycles (named for the Serbian scientist who discovered them) have resulted in an ice age and temporary dramatic expansion of the polar ice caps approximately every 100,000 years. Under ice age conditions, sea ice advances in the North Atlantic, which, through circulation systems in the ocean and air, can also cool the tropics. This cooling in turn leads to movements and weakening in the operation of key climate systems, such as the Intertropical Convergence Zone (where northeastern and southeastern trade winds meet around the equator) and the Indian summer monsoon system, which bring rainfall to many parts of the tropics today, resulting in drier conditions around the equator. Nevertheless, these broad solar models do not always explain all of the variation in the Earth’s climate over the last 10 million years, both in terms of extremes and frequency. In fact, temperature, precipitation, and CO2 at a given time and place also remained dependent on changing relationships between how heat and water circulated between the oceans, the atmosphere, and, crucially, land cover, including the greatest of earth systems engineers, the plants and forests themselves.13
Taking a broad-brush approach, we can identify some major climate- and atmosphere-driven changes in the varying fortunes of tropical forests and their grassland competitors over the past 10 million years. Starting in the Miocene (23 million to 5.3 million years ago), warm global temperatures and high CO2 concentrations had seen megathermal forests once again expand across much of Africa and Eurasia, reaching a maximum extent between around 21 million and 14 million years ago. From 10 million years ago, however, as CO2 concentrations declined, profound changes in vegetation occurred. Grasses practicing a novel form of photosynthesis (named “C4” after the number of carbons in the first sugar produced by the plant during the process), which was more efficient at absorbing CO2 and photosynthesizing in drier conditions and which dominates tropical savannah habitats as we imagine them today, joined existing “C3” grasses in the fight against warm, wet forests that saw the latter removed from Europe and Central Asia. Overall cooling and drying that had begun in the Eocene continued into the Pliocene (5.33 million to 2.58 million years ago), dealing another gradual but lasting blow to nontropical megathermal forests. As vegetation that had adapted to cooler, “temperate” climates expanded in the Northern and Southern Hemispheres, they retreated, regrouping near the equator. This was a new dawn. The age of tropical grasslands had begun, while other types of seasonally dry tropical forests (including those containing “deciduous” plants, which shed their leaves for part of the year) also began to emerge and diversify across the subtropics and tropics. Continued drying also saw deserts form in the most water-deprived areas of Africa, South America, and Asia, resulting in the layer cake of environmental variation we see as we travel from north to south across the globe today.14
While overall planetary cooling continued, from a mean annual surface temperature around 2°C warmer than today at the start of the Pliocene to as low as –6°C cooler than today during parts of the Pleistocene, the greatest characteristic of the Pleistocene epoch (2.58 million to 0.0117 million years ago) was actually the intensification of the impact of Milankovitch cycles. In the early Pleistocene (2.58 million to 0.77 million years ago) glacial cycles were more frequent (aroun
d every 41,000 years) but less extreme, while the middle and late Pleistocene saw the less frequent but more significant 100,000-year swings in global climatic states. In a tropical context, Pleistocene glacial cycles and a drying of the tropics have been argued to have led to an expansion of “savannah corridors” across different parts of Africa and Asia, perhaps even reaching between the two continents. Growing larger as glacial shifts became more extreme into the middle and late Pleistocene, these corridors would have provided ideal grassland conveyor belts for medium- to large-sized animals, which, unlike the usual dense tropical forest that had covered these regions, enabled them to traverse vast areas. In Southeast Asia, for example, some argue that such a “corridor” extended from southern China in the north down beyond the Wallace Line that runs through Indonesia, aided by the formation of a land bridge between mainland and island Southeast Asia, which appeared as growing ice caps sucked up water from the oceans. The end of wide-ranging savannah ecosystems was nonetheless inevitable, due to the fact that glacial ice ages, though longer and more extreme in this period, were not indefinite. The return of warmer, wetter, “interglacial” conditions broke up savannah extents, replacing them with classic tropical rainforest environments—hostile to many of the large-bodied grazing animals that had moved in during their absence.15
The last great glacial period, or Last Glacial Maximum (LGM), occurred between 26,000 and 21,000 years ago. Greater numbers of ancient archives of past climate and environment that relate to this more recent period allow us to determine the exact effect such a period might have had on tropical forests around the world in more specific detail than we can for earlier glacial periods. Although changes were not as extreme as in temperate areas, staggering drops in temperature of as much as 4°C to 8°C have been suggested for different parts of the tropics during the LGM. Given that our current rates of emissions predict a rise in tropical temperatures of anywhere between 2°C and 7°C between 1980–1999 and 2080–2099, this should highlight both the extreme nature of the LGM and also the disastrous biological consequences potentially awaiting us if we do not act. The LGM also made the tropics more arid. In South America and Africa, for example, it may have resulted in a rapid retreat of tropical rainforest to just a few small refugia within the usually vast Amazon and Congo Basin forests. Glacial declines in atmospheric CO2 may also have changed the structure of tropical forests, leading to smaller, more efficient leaves, open canopies, and increasing forest floor growth, as well as the increasing presence of mosaics of forests, woodlands, and grasslands. Whatever the cause, subtropical and tropical forest retreat between 26,000 and 21,000 years ago has been recorded in Africa, Southeast Asia, South Asia, New Guinea, Australia, and South America. The rate of forest re-expansion varied by region and was often complicated by other factors, including, in Southeast Asia, the impacts of rising sea levels on rainfall patterns.16
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