Geographical evidence for evolution
Before the 19th century, there was no explanation for the mysterious distribution of some of the plants and animals on the planet:
• Why is the fauna of North America and Europe similar on both sides of the Atlantic, while the fauna of South America and Africa so different?
• Why is the fauna of Australia so different from the fauna of the other continents?
• Why are there marsupials in America, like opossums?
• Why are there generally no large mammals on islands?
• Why are there camel fossils in America, but no modern-day camels exist?
Science should be able to clarify these questions with evidence.
Proximate location of Africa, South America, Madagascar, India, Australia, and Antarctica on the supercontinent of Gondwana 200 million years ago.
Similar Fossils in Distant Regions
Many examples of fossils of the same species have been found geographically far apart. This fact seemed strange until scientists understood that those regions were once part of the same continent.
The fossils of Cynognathus, an extinct tetrapod dating back 180 million years, have been found in both South America and Africa. This can be explained by the fact that South America and Africa were united 180 million years ago. The species populated adjacent areas that subsequently separated, explaining why its fossils appear on both continents.
Mesosaurus, Lystrosaurus, and Glossopteris are extinct animals and plants whose fossils can be found across the Southern Hemisphere. These organisms date back over 200 million years and once populated Gondwana, the supercontinent which separated into modern-day South America, Africa, Australia, India, and Antarctica.
Glossopteris, dating back 298 million years, is particularly interesting. The 19th century English geologist Eduard Suess studied the origin of this enigmatic plant fossil. Glossopteris was a small tree or shrub whose leaves resembled cow tongues. It dominated the land during the Permian era. Suess asked himself how Glossopteris fossils could be found across South America, India, Australia, and South Africa. Did Glossopteris evolve independently multiple times in multiple locations? Suess concluded that these continents must have been united at some point in geological history. He was the first to propose the existence of the supercontinent Gondwana.
Suess falsely believed there used to be dry land between the continents of the Southern Hemisphere. He argued that a great flood filled the lower elevations and separated the continents during the Permian era. It never occurred to him that the continents’ actual geographical locations could have shifted. The correct explanation, revealed by the theory of plate tectonics, was still decades away.
The history of these trees has a tragic component. When Robert Scott made his expedition to the South Pole, his team collected many Glossopteris fossils in Antarctica. Scott died along with all his men in 1912. The Glossopteris fossils were discovered when the bodies of Scott and his crew were found eight months later.
This was the first time scientists had evidence that Antarctica was formerly a part of Gondwana, a fact Suess could have never imagined. In addition to Glossopteris, many other fossils have since been discovered that evolved before Gondwana split into various continents 200 million years ago.
Cynognathus crateronotus fossil, Smithsonian Museum of Natural History.
Glossopteris fossils, Museum of Natural History, Houston, TX, USA
Marsupials in Australia
The movement of the continents can also explain the evolutionary history of marsupials. The oldest known marsupial fossils are more than 80 million years old and come from North America, not Australia. As the species evolved, they spread south and reached what is now South America, where marsupial fossils date back 40 million years.
Ten million years later, fossil evidence confirms that marsupials inhabited Australia. They diversified into roughly 200 different species, all still alive today. Scientists hypothesized that marsupials migrated across Antarctica to reach Australia, according to the continents’ known locations at the time.
To prove this hypothesis, scientists searched for marsupial fossils in Antarctica ranging between 40 and 30 million years old. At the beginning of the 21st century, a dozen species of marsupials with an age range between 35 to 40 million years were found on Seymour Island. This island was located between South America and Antarctica at the time.
Australian continent map
The Species of the Malay Archipelago
In 1856, Alfred Russel Wallace had already been in the Malay Archipelago for three years when he decided to study the birds native to each island. A fact caught his attention: “On crossing over to Lombok, separated from Bali by a strait less than twenty miles wide, I naturally expected to meet with some of these birds again; but during a stay there of three months, I never saw a single one of them, but found a totally different set of species, most of which were utterly unknown not only in Java but also in Borneo, Sumatra, and Malacca”.
East of Bali, an entire group of different species existed (and still do), more similar to those of Australia than to those of Indonesia. Tigers, monkeys, bears, and orangutans inhabit the area to the west of the narrow strait described by Wallace in his writings. Birds of paradise and marsupials, including tree kangaroos, inhabit the area east of the strait. They occupy the vacant niches created by the absence of monkeys. What mystery do these islands hold?
The Wallace Line is the imaginary line in the strait dividing Bali from Lombok and Borneo from the Celebes Islands. The sea is very deep in this area. The land to the east of the line is on one tectonic plate, and the land to the west is on another. In the past, when the sea level was lower, a single territory united the islands to the west of the strait. The eastern islands were also one single territory. However, since the water dividing the eastern and western lands was always very deep, the east and west territories were never united. The species had no opportunity to migrate from one side to the other, explaining why Wallace never found any eastern species on the western side or vice versa.
The Wallace Line reflects the location of the deepest region of the sea around the Malay Archipelago.
The shrimp of Panama
The isthmus of Panama separates the freshwater shrimp species on the Pacific coast of Panama from those on the Atlantic side. There are seven distinct species on the southern coast of the country. However, the species most genetically related to these seven separate species is found 40 kilometers away, on the northern Atlantic coast. How is this possible?
For millions of years, a single ocean existed at this location. Three million years ago, the isthmus of Panama emerged from the bottom of the sea because of volcanic eruptions, cutting off the water flow between the Pacific and Atlantic sides. As a result, the original freshwater shrimp species speciated into several new, closely related species that can be now found on both sides of the isthmus.
Shrimp
The Camelids of Different Continents
Camelids are a family of mammals, including camels and dromedaries in Asia and Africa, and llamas, guanacos, vicuñas, and alpacas in South America. Since the species distribution is not continuous across the planet, it deserves an evolutionary explanation. Where did camels and dromedaries originate? How did they get to Asia and Africa? How did their American relatives get to South America?
The continents of South America and Africa separated 200 million years ago when camelids had not yet appeared. This implies that the migration route could not have been from Africa to South America or vice versa. If camelids currently inhabit Asia, Africa, and South America, it was hypothesized that their ancestors evolved in North America and migrated to Asia and South America from there. Camelids in North America would be the link between Asian and African camelids and their South American cousins.
This hypothesis was confirmed by the discovery of 45-million-year-old camel fossils in western North America. Today, it is well known that the arrival of humans led to the extinction of the American camels some 15,000 years ago. The camels and dromedaries of Asia and Africa descended from these camels, who migrated through Alaska. Llamas, guanacos, alpacas, and vicuñas did not evolve in South America. They also descended from North American camels, migrating south when the North and South American continents joined together 3 million years ago.
Camels are one of the many species that fell victim to the first human settlers of North America. The evidence is staggering. When humans arrived on the continent 20,000 years ago, many large species were hunted to extinction. The woolly mammoth, the American mammoth, the ground sloth, the glyptodon (a beaver the size of a bear), the saber-toothed cat, the American lion, the cheetah, camel, horse, and many other large mammals all vanished from North America.
Since they evolved in an environment devoid of humans, these species were not afraid of the early settlers, making them easy targets for their spears. Today, the animals of the Galapagos Islands provide insight into how these extinct species reacted to the first humans when they came face-to-face with them 20,000 years ago. The native species of the Galapagos, particularly the birds, do not fear the presence of humans. They do not run or fly away when approached because they did not evolve in an environment where human predators hunted them.
Two camelids, the South American alpaca and the Arabian camel, or dromedary.
Sea lobsters and crayfish
Another evolutionary mystery that took more than a century to solve is the remarkable anatomical resemblance between saltwater lobsters and freshwater crayfish. Thomas Henry Huxley, a great friend and defender of Darwin, published a book in 1879 entitled The Crayfish. He discussed the great enigma surrounding these crustaceans. Although he never understood their origin, he predicted that they evolved in Australia, the continent with the most extensive variety of crayfish.
Some of the confusing findings included:
• Freshwater crayfish are found worldwide: Australia, North and South America, Europe, Asia, and Madagascar. How did they become so widespread if they cannot survive in saltwater? It was too far for them to swim from one continent to another.
• While crayfish and lobster anatomies are very similar, lobsters live in saltwater and cannot survive in freshwater.
It took more than 100 years for paleontologists, geologists, and geneticists to answer Huxley’s questions:
• The most recent common ancestor of lobsters and crayfish lived about 270 million years ago. This fact was discovered by geneticists using DNA analysis of two separate species. At the time, all the continents were united into one single landmass called Pangaea.
• Crayfish spread across the entire supercontinent of Pangaea through the river systems. Around 200 million years ago, Pangaea broke apart, and the modern continents began to form.
• 180 million years ago, crayfish evolved into two groups, those of the Northern Hemisphere and those of the Southern Hemisphere.
• Crayfish experienced what is known as adaptive radiation, a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms.
A change in the environment opens new environmental niches, allowing for adaptive radiation to occur.
Saltwater Lobster
Freshwater crayfish
Snails On Islands
In a letter dated September 28, 1856, Darwin wrote to his friend, the naturalist Philip Gossed: “The means of transportation…of land mollusks completely puzzles me.” A day later, he wrote to his cousin William Fox: “No theme provokes me more annoyance, doubt, and difficulty than the dispersal mode…towards the oceanic islands. Terrestrial mollusks drive me crazy.” Furthermore, that December, he wrote to Joseph Hooker: “I have been tormented and demonized by terrestrial mollusks for the last 15 months”.
Gastropods, or snails, represent 80% of the class Mollusca. They are one of the most successful groups of animals on Earth. They have been around for 500 million years, surviving several mass extinctions. They have adapted to almost all of the Earth’s habitats. About 35,000 species are known and estimated tens of thousands remain unidentified. Most are microscopic, smaller than any letter on this screen.
They may have spread across the planet by traveling on the bodies of dinosaurs or, later, “hitchhiking” on elephants, sloths, lions, or cheetahs. Unfortunately, this does not explain how land snails could reach islands far away from continents.
Darwin tried to answer this question by experimenting with snails. He placed them in containers of seawater and left them for up to 20 days. When the snails survived, he concluded that snails could be carried 1,000 km across the sea. In his book, On the Origin of Species, Darwin states that snails might have been transported across continents on the legs of birds.
Darwin’s several hypotheses about snails have proven to be true. Snails have been discovered on the feathers and legs of birds, much like tiny stowaways. Snails can attach themselves to the leaves of trees during a storm or the nesting materials of birds.
Different snail species
