It’s been a huge couple of weeks in evolutionary biology: Three teams of scientists, studying three separate periods spanning 450 million years make surprising discoveries about our bodies, our culture, and our ancestors.
The human eye is an amazing organ, the origins of which are ancient and shrouded in mystery. Complex eyes, from people to fish to insects, arose several times over the course of hundreds of millions years. Even before that, bacteria took advantage of light sensitive molecules to make their microscopic way in the world. It is believed that these original light sensitive chemicals ultimately evolved into light sensitive cells, resulting in a primitive form of sight in animals that lived as long as 555 million years ago. The earliest fossils discovered to date are from an ancient mollusk, Clementechiton sonorensis.
The origin of the human eye was believed to date back to about 350 million years ago. This belief was based on the nature of the connection between the eye and the brain. In humans and other complex, land-based animals, two sets of nerve cells connect the eye to the brain; each eye has a set of nerve cells attached to both the left and right hemispheres. Scientists thought this set up didn’t arise until life had migrated onto land and then was ultimately handed down to humans because of the benefits for depth perception and 3D vision.
They believed this because the great majority of modern fish use a different, simpler set up. Their eyes are connected to the opposite hemisphere of the brain by a single bundle of nerves. The zebrafish, for example, has been extensively studied and was believed to be an accurate model for how the eye has evolved in modern fish. “Modern fish, they don’t have this type of eye-brain connection,” explains Ingo Braasch from the Department of Integrative Biology in the College of Natural Science and a specialist in a very specific type of fish that uses a different model, more on that in a moment. “That’s one of the reasons that people thought it was a new thing in tetrapods.”
Now, however, researchers have discovered that the two nerve bundles connection existed at least 100 million years earlier than previously thought, originating before life moved onto land and literally rewriting the textbook on the subject. Scientists made this startling discovery by analyzing an ancient fish, the gar and determining it does, in fact, have the same two-bundle connection as humans complete with the cross-wiring.
As alluded to earlier, Braasch is one of the world’s leading experts on gar, a species of fishes that appears to have evolved more slowly than others. He was engaged by a team led by Alain Chédotal, director of research at Inserm and a group leader of the Vision Institute in Paris. “Without his help, this project wouldn’t have been possible. We did not have access to spotted gar, a fish that does not exist in Europe and occupies a key position in the tree of life.”
The study was completed with a groundbreaking technique that allows scientists to actually see the nerves connecting the eyes and brain. They applied this technique to several varieties of fish, including the zebrafish, gar, and another ancient species, the Australian lungfish. To their surprise, both the gar and the lungfish have the much more humanlike set up. Braasch explains, “It’s the first time for me that one of our publications literally changes the textbook that I am teaching with.”
The origin of the human eye now dates back at least a hundred million years prior to what was previously thought. This also means that some fish are closer to us than others, we share a more recent common ancestor with the gar and the lungfish than with other fish.
Fast forward about 450 million years, and another team of researchers is making startling discoveries about what our ancestors were able to accomplish with relatively small brains. This team has been studying the fossilized skulls of individuals who lived in Western Asia more than 1.7 million years ago. These hominids stood and walked upright, had some use of their hands, but their brains were half the size of modern humans and structured much more like our closest relatives, the great apes.
The team, led by Marcia Ponce de León and Christoph Zollikofer, use computer tomography and magnetic resonance imagery to map the inside of the early human skulls, creating what they call an endocast to analyze the imprint the brain leaves on the cranial vault. For this study, they created endocasts of five Homo erectus fossils discovered in Dmanisi, Georgia. It is believed that these hominids were the first of our ancestors to migrate from Africa, long before modern humans.
Dmanisi, Georgia itself is a treasure trove of fossils from a brief period around 1.7 million years ago. Located southwest of the capital at Tbilisi, there is a 10 acre paleontological site that sits at the crossroads of three continents, between Georgia’s arid east and humid west. The area was a migratory path for animals and situated between two rivers, making it what many have called a carnivore’s paradise.
Over the past 40 years, scientists have excavated at least 10,000 bones from 50 species of mammal including rhinoceros, elephants, deer, ostriches, and horses. There are also the super predators, a lean, coyote like wolf, two species of saber-toothed cat, lynx, bears, giant hyenas, a jaguar the size of a lion, and a giant cheetah. “I’ve excavated in a lot of places: Texas, Israel, Crimea, all kinds of sites,” explains University of North Texas geoarchaeologist Reid Ferring, who has worked at Dmanisi for nearly three decades. “Some of those sites change people’s perspective on the past. Dmanisi is one of those sites.”
By comparing the endocasts of the five fossilized skulls found at Dmanisi with other fossils, plus modern apes and humans, the team can draw conclusions about the structure and size of the brain. In this case, they had expected to find big brains, animals well on their way to becoming modern humans. “The prevailing idea up to now was that human-like brains evolved at the very beginning of our own genus Homo, so probably two million years ago or even earlier,” explains Zollikofer.
Big brains, however, were nowhere to be found. Instead, Zollikofer said, “There’s not much similarity with modern human brains.” This finding, however, stands in stark contrast with the behavioral complexity also uncovered at the site.
Previously, we believed that big brains are required for “human” traits like making tools and caring for the aged or infirm. The thinking was that our brains got bigger and, as a result, our society became more sophisticated, but these hominids had “surprisingly primitive, ape-like brains.” Still, according to Ponce de León they “ventured out of Africa, produced a variety of tools, exploited animal resources, and cared for elderly people. These people with their small, ape-like brains were able to master cognitively demanding tasks. This is really astounding. They provide a completely new perspective on what these behaviors mean in terms of brain evolution.”
Fast forward another 1.65 million years, and yet another team of scientists is discovering more about humanity’s migration to Europe and our interactions with our cousins, the Neanderthals.
The subject of this study was the DNA of humans who lived between 45,900 and 42,600 years ago. Interestingly, we have previously studied the DNA of humans before and after that period, but until now there has been a dearth of genetic information for that relatively short span of years. A team lead by Mateja Hajdinjak and evolutionary geneticist Svante Pääbo from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany was able to analyze a tooth and fragmentary remains of three individuals found in the Bacho Kiro Cave in Bulgaria.
They were able to determine that these early humans had close to twice the Neanderthal DNA as previous specimens. Earlier studies had indicated that modern non-Africans generally have about 2% Neanderthal DNA. The new study found 3.4% to 3.8% and the scientists believed they all had recent ancestors who were Neanderthals, only 6 or 7 generations removed. The scientists also believe the interbreeding between humans and Neanderthals occurred in Europe, not the Middle East where it was commonly thought Homo sapiens and Neanderthals mixed. “We saw these huge chunks. It was completely amazing,” explains Hajdinjak.
Unfortunately, the Bacho Kiro group appears to have died out, and none of them are related to modern day Europeans, meaning modern Europe was populated by another separate group, that either arrived later or didn’t interbreed with the Bacho Kiro population. The Bacho Kiro’s story doesn’t end there, however. While they are not related to contemporary Europeans, they are related to contemporary East Asians and Native Americans. The scientists surmise that they represented a Eurasian population that ultimately vanished in Europe, but survived elsewhere.
The implications paint a picture of a much more dynamic prehistoric world, one where humanity arose out of Africa in many separate waves, interbred and exchanged with the local populations of Neanderthals, covering huge swaths of ground, exploring, learning, and living together side by side.
Only two things are certain at this point: First, as our knowledge expands, the textbooks will be continually rewritten. Technology is rapidly improving our ability to analyze and understand aspects of the ancient world, either preserved in fossils or our own bodies as DNA and organs. We have barely begun this journey. Second, it should not be surprising if there are more surprises in store. It is one of the limitations of our relatively short life spans and cause-and-effect view of the world that we tend to see things in static, neat little groupings. We put labels on animals and ourselves, believing they describe them accurately. A fish is a fish, or a Neanderthal is a Neanderthal.
This is an illusion. The real world is much more dynamic and interesting. A fish is still a fish in one sense, but in another it takes on a closer connection to ourselves than its own nearest relatives. Our big brains separate us from other animals, but not as much as we think; particularly in regards to behavioral complexity, you can do more with less. The Neanderthals were a related species, but they didn’t simply die out, they live on in us today, and our own direct ancestors conquered the world long before we had a name for it.