The birth of the Amazon

Understanding how the world's largest river was formed helps scientists explain the unusual abundance of plants and animals in the Amazon rainforest

By Karina Horn, Scientific American. Published in Issue 24, August-September 2006

A viewer looking at the Amazon River from above cannot help but notice that the landscape is dominated by water, and not just the meandering main channel. The river, which stretches from the highlands near the Pacific Ocean in Peru to the Atlantic coast of Brazil, 6,500 kilometers away, overflows during the rainy seasons and floods vast areas of forest. Throughout the year countless lakes emerge and cover its floodplains.

The river feeds a total area of ​​2.5 million square kilometers of the world's most diverse rainforest. However, until recently researchers had no idea how long this close connection between the forest and the river actually existed. Because of the difficulty of reaching these remote areas, now called the Amazon Basin (or Amazonia), the long-accepted theories about the early days of the river and the rainforest remain conjectural at best.

In the last 15 years, new opportunities have arisen to study the rocks and document the fossils in the area, and they have allowed researchers to finally create a more complete picture of the history of the Amazon. The findings show that the birth of the river was a complicated process that lasted millions of years, and that its development greatly influenced the evolution of the local plants and animals. Many researchers now claim that the young river fed a large number of lakes that were connected to each other in the center of the continent, before carving out a direct route to the Atlantic Ocean. This dynamic wetland created perfect conditions for marine and terrestrial life to thrive, much earlier than previously thought. The new explanations also answer the question of how creatures that usually live only in the ocean, including dolphins, now thrive in the inland lakes of the Amazon basin.

Precipitation numbersAnswers to the questions of how and when the Amazon was formed are essential to understanding how the river influenced the evolution of life in the Amazon Basin. Before the early 90s, all geologists knew was that powerful movements in the earth's crust shaped the Andes of South America and the peaks of high mountains elsewhere (including the Himalayas and the Alps) 24 to 5 million years ago, a period in Earth's history called the Miocene . These dramatic events caused the birth of new rivers and diverted old rivers from their course in Europe and Asia. Experts assumed that South America was no different, but the exact nature of the changes that took place there and their timing remained unknown.

When I began researching this mystery in 1988, I suspected that the best evidence of the early Amazonian environment would be found in the large deposits of mud, sand, and plant remains that sank in the basin where the great river now flows into the Atlantic Ocean. It was not an easy challenge to get to these deposits, which were weathered a long time ago and became mud rocks, sandstones and other types of rocks. The great jungle that spans eight countries, each with its own laws, does not reveal its secrets easily. And the rocks that form the basin, rarely protrude above the surface, are often located near inaccessible streams and are usually covered with thick vegetation.

Along the hundreds of miles of waterways that my field guide and I surveyed, in Colombia, Peru, and Brazil, we encountered only a few dozen large rock deposits. Most of the time we had to use a machete to remove the foliage - once we surprised a huge green anaconda snake and another time we uncovered jaguar tracks. And even then, we could only reach the upper layers of the thick rock formations, which in some places reach a depth of one kilometer below the ground.

After completing the initial fieldwork, my first conclusion was that the Amazon River did not exist until about 16 million years ago, the beginning of the period geologists call the Middle Miocene. Most of the rocks we found that dated to earlier periods were made of reddish clay and white quartz sand that were clearly formed by the erosion of granite rocks and other light-colored rocks in the interior of the continent. This composition suggested that the ancient waterways in the region emanated from the heart of the Amazon basin. My conclusion - which was later confirmed by other researchers - was that during the early Miocene, the rivers came out of a low mountainous area in the interior of the continent and flowed in a northwesterly direction, some of them eventually spilling into the Caribbean Sea.

Shortly thereafter, the Amazonian landscape changed considerably as a result of violent tectonic events that began to push up the northeastern Andes. In the rock record, the red and white sedimentary rocks disappear about 16 million years ago. In their place we alternately find blue-turquoise, gray and green clay, brown sandstone and fossilized plant material called lignite. It was clear that the origin of the dark mud and sand particles was different from that of the light granite rocks. The unique layering pattern in the sedimentary rocks showed that the water that layered them no longer flowed north but east. My guess was that the rising mountains in the west changed the drainage patterns and sent the water eastward towards the Atlantic Ocean.

Later analysis of the sediments, conducted at Wackeningen University in the Netherlands, supported the idea and demonstrated that many of the brown sand grains were indeed fragments of dark slate and other typical rocks that began to erode as the young Andes rose. Moreover, some of the pollen grains and spores I found in the clays and lignite were of conifers and thorny ferns that only grow in high mountains. These pollen grains are very different from the fossilized pollen grains found in the earlier Miocene sedimentary rocks, which are derived from plants that only grow on the interior plains. Additional reinforcement of my conclusions came from drilling in a certain rock formation in Brazil, the only one that yielded rock cores that include continuous and complete documentation of layers, including the transition layers from reddish clay to the blue and brown sedimentary rocks.

And finally, scientists had solid evidence of the birth time of the young Amazon River. But it soon became clear that the river did not reach its full glory until a long time later. In 1997, David M. Dobson, now at Gulliford College in Greensboro, North Carolina, and his colleagues discovered that the sand grains from the Andes, which I found in the Amazon basin, began to accumulate along the Brazilian coast only about ten million years ago. The meaning of this finding is that it took the river at least six million years to develop and become the connected drainage system that crosses the continent today. The study of the geological changes that took place during this period of transition is now shedding light on the origins of the area's mysterious fauna today.

Doubts about the maritime connectionFor decades, the dominant hypothesis about the Amazon basin during the Miocene claimed that a shallow sea covered the area for most of the period. The discovery that the Amazon River took millions of years to mature did not contradict this view, because the possibility that the sea blocked the path of the young river to the Atlantic remained open.

Adherents of this assumption say that a long connection with the open ocean also explains how dolphins, manatees, trigons and other sea creatures arrived at the heart of the continent. Then, when the sea receded, they developed resistance to fresh water. This is why they still thrive today on marine "islands" in the forest. Researchers in the south of the continent have presented convincing evidence that shallow sea conditions also prevailed in mainland Argentina during the Miocene.

Such arguments are tempting to believe in the sea scenario, but my colleagues and I discovered some evidence that refutes the idea that such a marine link dominated the landscape for a long time. Instead, we believe that the Middle Miocene rock samples I collected, representing the period from 16 to 10 million years ago, are the remnants of a predominantly freshwater environment.

One of the most striking features of the Middle Miocene rock layers is their regularity. At the very beginning I thought that this pattern developed when different types of sediment were deposited in the different seasons, dry and rainy, over the years, a common pattern in wetlands fed by small rivers. During the dry season, particles of soil and plant matter slowly settle to the bottom of shallow lakes and marshes, eventually forming the blue clay and lignite. During the rainy season, stronger currents that washed down from higher places in the west—perhaps even the young Amazon River itself—could have brought the brown sand with them. Moreover, the minerals characteristic of the Andean highlands appeared only in the sandstone layers.

Some researchers interpret the same sedimentary rocks differently. Mati Rasnan from the University of Turku in Finland and his colleagues claim that the alternating types of precipitation record the rise and fall of ocean tides, which could have significantly altered the shoreline of the shallow sea or the estuary that they believe covered the Amazon basin at that time. The rising tide may have brought the sand, while the mud and plant material sank in its retreat. It is important to add that tides may also occur in large freshwater lakes.

The strongest evidence that the area was covered by fresh water during the Middle Miocene comes from the many fossils trapped in the rock. Frank P. Veslin, today at the Naturalis Museum in Leiden in the Netherlands, accompanied me to Colombia in 1991. During the tour, he discovered a wide variety of oysters and snails fossilized in the Middle Miocene rocks. Vaselin conducted a detailed taxonomic study of these clams and snails, based on earlier work by Patrick Nuttall, then at the British Museum in London. He found that most of the fossils, representing about seven million years and dozens of sites in the Amazon basin, are of clams and snails that were adapted to life in freshwater lakes. Only a few species could survive in a completely marine environment. A prolonged connection to the sea like the one proposed by Rasnan would not have allowed such an adaptation during the flooding period. In the same way, a salt sea would wipe out species that live in fresh water and that evolved before its existence.

A few years later, in 1998, Hubert B. Pohnhoff of the Free University of Amsterdam, Veslin and colleagues reached the same conclusion based on the chemical signature of the fossilized shells. Clams and snails slowly grow their shells around their bodies from carbon, oxygen, strontium and other elements dissolved in the water that surrounds them. Hence the chemical composition of the shell layers serves as a record of the composition of the water during the mollusk's life. It is possible to distinguish between strontium dissolved in salt water and that dissolved in fresh water according to differences in the distribution of its isotopes, which differ from each other in the number of neutrons in the atomic nuclei. It is therefore possible to use isotope ratios as a measure of water salinity.

To the surprise of many scientists, the strontium isotope signature was relatively constant over long periods of time and over large areas where the clams and snails were examined. The find showed not only that their habitat was mainly fresh water, but also that the swamps formed a very large and connected body of water. According to current estimates, it covered 1.1 million square kilometers and was twice the area of ​​the Great Lakes of North America, marking it as one of the largest and longest-lived lake systems to have ever existed.

short invasionsDespite the mounting evidence against a prolonged salt-sea connection in the Miocene Amazon basin, the strontium signatures in oysters and snails revealed that this massive lake system occasionally experienced salinity increases. It is known that the sea level during the Miocene was higher than the sea level today, and therefore it is likely that the rising Caribbean Sea broke south along a narrow continental passage. And indeed, plant and animal fossils confirmed short periods of contact with the ocean. Microscopic marine creatures and pollen from mangroves—trees that thrive in salty seawater—appeared in my rock samples, but only rarely and for short periods of time. In conclusion, the evidence shows that the Amazon basin was flooded at least twice during the existence of the vast swamp, 16 to 10 million years ago.

According to the best estimates, each of the marine floods lasted thousands of years, not millions of years. And although they never compared the salinity of the lake to that of the open ocean, they nevertheless allowed animals from the ocean to penetrate into the heart of the Amazon basin. However, detailed investigations of the history of certain creatures suggest that the last great sea connection was interrupted when the Amazon was still in its infancy, long before the ancient swamps gave way to the river that crosses the continent today. Molecular studies conducted by Nathan R. Lovejoy, now at the University of Toronto at Scarborough, for example, have shown that the Amazonian trigons, very close to those living in the Caribbean Sea, migrated inland about 16 million years ago.

Dolphin studies have drawn similar conclusions. Insa Ksens and her colleagues from the Free University of Brussels, came to the conclusion in 2000 that the pink river dolphins living today in the Amazon region are the remains of marine dolphin species that were common in the early Miocene but became extinct shortly after. This shows that the dolphins found there now have adapted to fresh water. Also, Eulalia Banguara-Hinestroza from the University of Vela in Colombia recently distinguished two genetically distinct groups of Inya dolphins, one in the Amazon and one in Bolivia, demonstrating that these groups separated a considerable time ago; Such a separation would not have occurred while the sea connected these areas.

The age of the rainforestThe accumulating clues that the Amazon River basin during the Miocene was more like a lake than a sea are forcing scientists to reexamine the history of the rainforest. One of the main theories about the origin of biodiversity in the Amazon claimed that it was created following the ice ages of the last million years. The formation of arid conditions, similar to those created by the ice ages in the northern part of South America, is one possible way in which the ancient rainforest could have shrunk and fragmented into small, isolated habitats. Many evolutionary biologists assume that such separation is essential to the creation of biological richness: the theory goes that when small areas of a large habitat are cut off from each other, adjacent populations of a given species stop interbreeding. Over time, this reproductive isolation allows one population to differentiate genetically from the others, to the point of creating a completely new species. As the zones reconnect during warmer periods, the species remain distinct even when their habitats overlap.

But again, the new findings tell a different story. The same evidence that discovered the lake's ecosystem also shows that the host of modern animals and plants in the Amazon basin thrived already millions of years ago. For example, the complete list of types of plant pollen that my assistants eventually recovered from the Miocene rocks represents an amazing diversity of plants. I identified 214 species. Many more species have been removed from the list because I found only one example of them. Most of the most common pollen grains in the samples were from flowering plants that grow mostly on river banks, and their diversity is similar to today's forest plants. In any case, prolonged seawater inundation, as others have suggested, would have severely limited the opportunities for these terrestrial species to take over the area until a much later time. This finding casts further doubt on the long-term existence of a sea in the Amazon basin.

New research on molluscs supports the results obtained from the pollen grains, and suggests that the Miocene climate could have supported a diverse rainforest. In this study, Ron JJ Kandorp from the Free University of Amsterdam examined the growth layers in the shells of oysters and snails from 16 million years ago, this time by testing oxygen isotopic signatures, which record the amount of rainfall. The growth layers show an alternating alternating pattern, very similar to the patterns known from modern Amazonian shells. It is known that in modern shells, the layers change alternately with the change of dry and rainy seasons on which the forest depends. Although the world was a warmer place during the Miocene, the presence of almost identical oxygen signals in the ancient shells shows that the climate supporting a rainforest ecosystem already existed when these oysters and snails lived - long before the ice ages of the last million years.

The sex explosionIn light of the new evidence, more and more scientists agree that the swamps of the Amazon basin during the Miocene were the cradle of biological diversification that resulted in an evolutionary explosion. The uplift of the Andes started the process by causing the birth of rivers, including the Amazon, which fed a vast swamp that dominated the Amazon basin for nearly seven million years.

Marine invaders have reached the area on several occasions. The environment of the interconnected freshwater lakes was discovered after the invasion as a perfect habitat for the new marine animals, such as oysters and snails, which multiplied and developed into a rich variety in an incredibly short period of time - perhaps even within a few thousand years. This environment was also perfect for a type of small crustaceans called ostracods. Fernando Muñoz-Torres, of the Colombian oil company Ecopetrol, discovered that clams, like oysters and snails, also went through a turbulent differentiation phase during that time. Both the shallowness of the lakes and canals, and the partial isolation of some areas, apparently increased the rate of species creation.

Then, when the connected lake system gave way to the mature Amazon River, most of the freshwater molluscs and clams that needed quieter lake conditions became extinct. However, the changing landscape at the same time allowed a wider variety of terrestrial animals and plants to develop.

One of the most encouraging discoveries from current geological research is that the animals and plants of the Amazon are extremely resistant. During the 22 million years of the rainforest's existence, it has survived and even thrived despite enormous changes in the landscape: the rise of the eastern Andes, the birth of the Amazon River and the inundation of seawater. Do we dare to hope that this resilience will help the Amazon basin also meet the challenges that we humans put before it?
 Overview / Fresh Start
For a long time, many scientists assumed that a shallow sea covered the Amazon basin for most of its history, and that the current biodiversity of the rainforest developed only recently.
New research shows, however, that rain hair was already thriving when the Amazon River formed about 16 million years ago.
It now appears that the evolution of the modern forest was significantly influenced by the long and dramatic birth process of the river itself, which began to flow uninterrupted across the continent only about ten million years ago.

Water to sea?One of the most mysterious mysteries in the modern Amazon basin is the existence of dolphins, trigons and other animals of marine origin that inhabit the freshwater and murky waters of the rainforest. An old theory holds that a shallow sea crossed South America from north to south for most of the Miocene epoch, from 25 to 10 million years ago, and that these animals are descendants of ancient marine ancestors who migrated to the region via this waterway. Then, when the sea receded, the species adapted to fresh water.

Notes from the fieldResearch in the Amazon region is not easy at all, as my first days there in 1988 demonstrate. I hoped to find and analyze sedimentary rocks buried under the layer of soil and thick vegetation to clarify the development of the area.
Since the rocks I was looking for were hundreds of miles apart, I found myself wandering the banks of the Amazon's many tributaries for weeks at a time. My base camp was on a site that used to be used as a prison in Araquara, a place so remote that escaped prisoners were thought to have no chance of survival. Fortunately, my Indian field guide, Anibal Matefi, helped me.
On one memorable day, Anibal and I rowed a boat for hours along the Caqueta River in Colombia. Each bend in the river seemed to me the same as the last, but Anibal had spent his whole life in this part of the jungle and he knew exactly where we were. We stopped at an abandoned house to hang our hammocks for the night. The next day we were supposed to look for exposed sedimentary rocks by the Epphoris River, in an area cut off from Kuta by some scary waterfalls. The raging water forced us to abandon the boat and walk on foot, with the engine in our hands.
Anibal and I walked across the hills towards Apforis and there we borrowed a smaller boat from the local Indians. Another group gave us shelter for the next few days while we searched for the coveted rocks. Our hosts were friendly at first, but soon seemed unhappy with our presence. The reason quickly became clear. The tribe was visited by another non-Indian and we suspected that he was there to find shelter. Colombian guerilla forces were active in the area at the time, and he was probably one of them. We were happy to leave having achieved our goal: collecting many kilograms of black clay and other ancient sedimentary rocks.
Of all the journeys I made, the journey to Epphoris was the most difficult. But while we dug diligently during the day to find exposed rocks and swung in hammocks at night, time seemed to stand still in that remote place - and the high-tech world existed in a parallel universe.

on the notebookKarina Hoorn is a geologist and plant pollen expert at the Institute for Biodiversity and Ecosystem Dynamics in Amsterdam. She received a PhD from the University of Amsterdam in 1994 and an MA in Science Communication from Imperial College London in 2004. She studied rivers in the Amazon basin, the Andes, the Himalayas, and the Sultanate of Oman to identify the effects of these depositional environments on local vegetation. In addition to engaging in scientific research, Horn currently reports on new technologies for locating and producing oil and gas for the Shell oil company in the city of Ryswick in the Netherlands.

The changing shape of the Amazon basinThe vast interior regions of northern South America, now known as the Amazon Basin or Amazonia, have undergone at least three major landscape changes over the past 25 million years. Many researchers today agree that the Amazon basin was covered by sea water only occasionally during the period. According to this new view, the uplift of the Peruvian Andes sent water eastward earlier than previously thought, and the young Amazon River fed one of the world's largest connected lake systems for millions of years before eventually emptying into the Atlantic Ocean nearly 6,500 kilometers away.

25 million years agoNear the beginning of the Miocene era, the Amazon River and the northeastern Andes did not exist. The main waterways of the Amazon basin originated in an area of ​​low mountains in the heart of the continent and flowed towards the northwest, some of them finally spilling into the Caribbean Sea.

15 million years agoIntense tectonic activity raised the northeastern Andes to almost a quarter of their height today. As a result, they blocked the rivers facing the northwest and flowed water on their eastern slopes. One of these streams became the young Amazon River, which fed vast swamps that slowly spread eastward.
currently
The river reached its current length about 10 million years ago, when it made a direct route to the Atlantic Ocean, apparently following another tectonic uplift of the Andes. When the river emptied into the ocean, it drained many of the lakes that dominated the Amazonian landscape for a long time and began to flow sediments off the coast of Brazil. These sediments now form one of the largest submarine alluvial fans in the world.

New and sweet ideasThe evidence buried in 16- to 10-million-year-old rocks—which include layers of sediment, plant pollen grains, and fossilized clams and snails—suggests that brackish water conditions rarely prevailed in the Amazon basin during the mid-Miocene. Instead fresh water was common.
Cyclic layers of fossiliferous sediments from the Middle Miocene are typical of stratification in the bottoms of shallow marshes fed by small rivers. During the rainy seasons, strong currents carry many sand particles (thin brown layers) to the lake bottoms. In drier seasons, mud particles are common. As they sink slowly, they form clay deposits (blue layers) on top of the sand.
The most common pollen grains in Amazonian rocks from the Middle Miocene came from flowering plants, such as Caesalpinoideae and Bombacaceae. These plants are known to grow almost exclusively along the banks of tropical rivers. Rare appearances of mangrove pollen grains and marine microscopic creatures in the same sedimentary rocks confirm that saline waters probably invaded the area for only short periods of time.
Freshwater clams and snails, including Sioliella and Pachydon, provide the vast majority of Middle Miocene shells discovered at dozens of sites throughout the Amazon basin; Only a few of the species found could survive in salt water.
Oxygen isotopes in the shells of oysters and snails, such as those measured in the 16-million-year-old shells of the Diplodon oyster, clearly show that the oysters lived in a tropical flood lake. More specifically, the growth streaks in Diplodon, the layers of the shell that oysters grow using the chemical elements dissolved in the surrounding water, show an alternating pattern of enrichment and depletion of the less common oxygen isotope 18. Such alternating patterns, also found in the modern Amazonian clam Triplodon, mean that the animals experienced the wet and dry seasons typical of tropical rainforests; The curves would be much flatter if the oysters lived, for example, in a salty sea.

And more on the subject
Origin and Evolution of Tropical Rain Forests. Robert J. Morley. John Wiley & Sons, 2000.

Seasonal Amazonian Rainfall Variation in the Miocene Climate Optimum. Ron JG Kaandorp, Hubert B. Vonhof, Frank P. Wesselingh, Lidia Romero Pittman, Dick Kroon and Jan E. van Hinte in Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 221, Nos. 1–2, pages 1–6; 2005.

New Contributions on Neogene Geography and Depositional Environments in Amazonia. Edited by C. Hoorn and HB Vonhof. Journal of South American Earth Sciences, Vol. 21, Nos. 1–2 (in press).