October 2024

Where's the evolution?

Today, the continents of Africa and South America are separated by an entire ocean, making a walk between them impossible. But that wasn’t always the case. Over at least the last 2.5 billion years, the parts of Earth that are exposed as land have shifted as sea levels have risen and fallen, and as Earth’s crust has reconfigured and remade itself through plate tectonics. The theory of plate tectonics proposes that the rocky outer layer of the Earth is composed of a series of gigantic plates that fit together like a jigsaw puzzle. These plates slide (very, very slowly!) over the layer beneath them. Where the plates move towards one another, they may collide and push both plate edges up, forming a mountain range (the Himalayas formed this way), or one plate may slide underneath, lifting the other plate up (the Andes formed this way) and forcing the lower plate down to be melted into magma. Where the plates move apart, lava flows up and forms new crust, building on the edges of the two plates.

Through billions of years of incremental plate movement, Earth’s exposed landmasses have migrated around the globe, cycling through periods in which they converge into a single supercontinent and then break up again into multiple distinct continents. Earth’s most recent supercontinent, known as Pangaea, existed from about 335 million years ago until 200 million years ago, when it began to break apart. First, it split into two landmasses (shown on the second map below). Laurasia was composed of modern-day north Asia, Europe, and North America, and Gondwana was made up of modern-day South American, Africa, Antarctica, India, Australia, and Arabia. By 120 million years ago, when the dinosaur trackways were laid down, Gondwana had broken up a bit, but South America was still tucked into the hollow of Africa (as shown on the third map below) – and so land comprising modern-day Brazil abutted that which we now recognize as Cameroon. Land animals would have been able to walk from one to the other.

Finding matching dinosaur footprints in Cameroon and Brazil makes perfect sense in light of the theory of plate tectonics. Indeed, when Alfred Wegener proposed the idea that the continents are mobile more than 100 years ago, he marshalled just this sort of evidence to make his case. He argued that the distribution of a wide range of organisms – including earthworms and the fossils of an extinct tree – all fit with the idea that continents move.

Of course, scientific theories that become widely accepted, as plate tectonics and evolution have, aren’t supported by just one line of evidence. These theories are supported by and help make sense of a wide range of phenomena and observations. Wegener, for example, pointed out that the idea that Earth’s continents had all once been joined together also helped explain their shapes, the distribution of coal deposits and other geologic features, and the evidence suggesting that the ancient climates of Earth’s landmasses differed substantially from those of today. Evidence collected decades later showed that plate tectonics also explains the patterns of magnetic polarity recorded in Earth’s rocks. And still later, when underwater mountain ranges in the middle of the ocean and seafloor spreading were discovered, it all made sense if plate tectonics were true. Today, satellites can measure exactly how much and how fast Earth’s continents are moving. The theory of plate tectonics is supported by and helps explain all these diverse lines of evidence; they all fit. The same is true of evolutionary theory. The genetic sequences of different organisms, their physical and behavioral traits, the fossil record, and a wide variety of experimental results, all make sense if different species share common ancestors and diverged through evolutionary processes like natural selection.

Explaining a wide range of phenomena is part of what makes plate tectonics and evolution good theories, as opposed to good hypotheses. In casual conversation, we might use the words theory and hypothesis interchangeably to mean a guess. But in science, hypotheses and theories are not mere guesses. They are potential scientific explanations, usually based on prior experience, background knowledge, observations, and logical reasoning, that differ in scope. Hypotheses apply to a fairly narrow set of phenomena, while theories are broader explanations that integrate and generalize many hypotheses.

To be accepted by the scientific community, theories must be supported by many different lines of evidence, and they must continue to explain new evidence and make new predictions that hold true. When the scientists behind the dinosaur track research dated the Cameroonian prints to 120 million years old – a time when our understanding of plate tectonics tells us that Africa and South America were still part of the same landmass – they wondered if these prints might be related to South American dinosaur footprints they’d heard about. As we’ve just seen, a little more digging revealed that they are closely connected, despite the long flight that separates them today.