When we look at the world around us, we see a wonderous variety of living things—with fins, fronds or feet, living in the deepest ocean or the driest deserts, eating seeds or salmon or sunlight.
The term “missing link” refers to a question people often ask about how species evolve into all these different forms. Shouldn’t there be an intermediate creature between an ape and a human, for example?
Experts consider this an outdated term. Instead, think of evolution as a tree, not a chain. That is, not every animal species directly evolves into another species; just as dodo birds and T-rexes went extinct without evolving into something else, some branches on the evolutionary tree just end.
That means that humans and apes both evolved separately from a common ancestor millions of years ago, as opposed to a single, direct chain as one species evolved into another.
That said, scientists have found many fossils with attributes of creatures that lived close to key branching points in the tree. For example, University of Chicago paleontologist Neil Shubin discovered a very ancient fish fossil named Tiktaalik that had features of our limbs within its fin skeletons—from the time that animals began to move from living solely in oceans onto land.
What are “missing links” of evolution?
People use the term “missing links” to refer to species in the fossil record that show evidence for how living things evolved the wildly varied features we see today, such as lungs, wings or feet.
Hundreds of millions of years ago, all life existed in Earth’s vast oceans. Scientists have traced how species evolved to walk on land and take to the skies by finding fossils, examining DNA and running experiments.
“At first glance it seems so utterly impossible. How did fish evolve to walk and breathe and feed on land?” said Prof. Neil Shubin. “But the reality is, as we discover fossils, as we look at genetics and DNA and embryos, that great transition becomes not only possible but highly likely.”
In 2004, Shubin led a team that discovered Tiktaalik, an ancient fish that lived 375 million years ago and had features in between fish and modern-day land animals. This was a pivotal point—when life first began to venture out of the oceans and onto land.
Piece by piece, scientists keep assembling more data to trace the evolution of life over time, creating a complex diagram. They are constantly re-analyzing and testing to better understand how evolution works.
One thing that we can see is that evolution is not an orderly progression of one species into the next. Random chance and natural selection both play a role in which species persist and which die off. For example, a species that was perfectly adapted to living in a forest might die out if the area happens to flood over time and becomes a swamp.
The whole thing is more like a twisted, tangled tree than the steady rungs of a ladder. This is why the term “missing link” isn’t considered very useful in modern evolutionary science.
“With the growth of evolutionary biology came recognition that the history of life is tree- shaped,” said UChicago Prof. Michael Coates. “Today’s species represent the tips of living shoots and twigs, while the fossil record documents the dead branches and amazing shrubbery of long-lost evolutionary radiations that failed to make it through to the present day.”
The mission of scientists like Coates, Shubin, and their colleagues, is to reconstruct this tree of life.
“We can use living and extinct species to look at changing patterns of biodiversity through deep time,” said Coates. “Every day we fill in more details of this fascinating picture.”
Why is it so hard to find missing link fossils?
Finding fossils at all is hard. Only a tiny percentage of all living organisms on Earth will wind up as fossils; most of us are folded back into the grand circle of life.
Usually it takes an unusual circumstance, such as a sudden flood that buries bodies under sediment, or the famous tar pits of La Brea, to result in fossils. Weather conditions affect the chances too—skeletons in dry deserts are more likely to survive than those in tropical rainforests, for example.
Some body types don’t fossilize easily, either. For example, soft-bodied animals like jellyfish are rarely preserved, and the same is true for animals with delicate bones, such as birds. And even if they do initially persist, fossils may get crushed under tectonic plates, melted by volcanoes or worn away by water over time.
But by piecing together what we do have and comparing that across the entire scope of fossils across the world, we can get a very good sense of how life on Earth has evolved over time.
How do scientists find missing links?
Scientists have multiple ways to understand the pattern and history of evolution. These include:
- Comparing anatomy. By closely examining the bodies of different creatures, both modern and fossilized, scientists can find clues about evolution. For example, a whale’s flipper contains the skeleton of a five-digit forelimb much like we see in other vertebrates, including us; this can help place it in the tree of life relative to other species.
- Examining DNA and genetic material. The way all species evolve is by tiny changes to their genes over time. By sequencing the DNA of many modern animals—and even sometimes pieces recovered from ancient skeletons—and comparing them, scientists can put together a picture of how species changed over millions of years.
- Running experiments. Scientists can use experiments in the laboratory to explore how evolution unfolds. For example, they might use fruit flies to see how changes are passed down through generations, or how fast a new species can emerge. They can also run computational simulations to, for example, re-create how proteins might have evolved.
- Finding fossils. Fossils are tricky to find, and searching for a specific fossil is even harder. But paleontologists can make educated guesses about where to search, based on geological knowledge and where other fossils have been found. These offer invaluable insights into how organisms change over time—for example, inside ancient fish fossils, we can see structures that likely evolved into teeth.
What is Tiktaalik?
In 2004, UChicago Prof. Neil Shubin led a team that traveled to the Arctic and discovered the fossil of a fish that had both fins and also the beginnings of feet. These would have allowed it to move in shallow water and perhaps on land.
“What we saw gradually emerge from these rocks during the fall of 2004 was a beautiful intermediate between fish and land-living animals,” Shubin wrote in his bestselling Your Inner Fish.
In consultation with Indigenous Arctic peoples, they named the species Tiktaalik roseae. ‘Tiktaalik’ means a shallow-dwelling fish in the Inuktitut language.
The ancient Tiktaalik lived about 375 million years ago. Before that era, all animals lived in the water.
The fish probably spent most of its time in shallow waters of streams or ponds, but it may also have flopped around on the mudflats along the banks. This could have been an advantage given that it likely had many predators and competitors in the water but none at all on land.
Studying Tiktaalik, Shubin and his team were surprised to discover the creature had a shoulder, elbow and wrist composed of the same bones as an upper arm, forearm and wrist in humans.
“So every time you bend your wrists, every time you shake your head, you can thank these fish living in streams 375 million years ago,” Shubin said. “It shows how our bodies contain layer after layer of history, and that history is so important to understand why we look the way we do.”
Tiktaalik might or might not be a direct ancestor of ours; that honor might belong to one of its cousins instead. But it’s one of many similar fish that evolved around that time and can show us how evolution works.
What are other examples of missing links that have been found?
A classic “missing link” fossil is Archaeopteryx, an animal that lived about 150 million years ago and has features of both modern-day birds and reptiles.
Discovered in 1861, it remains an iconic example of the links between dinosaurs and modern-day birds, even as scientists have discovered many other fossils of dinosaurs with bird-like characteristics like feathers and hollow bones.
Scientists are finding more “missing links” all the time.
For example, UChicago Prof. Michael Coates helped reconstruct Acanthostega, a 360-million-year-old fossil exhibiting characteristics of both fish and amphibians—in many ways the evolutionary counterpart to Tiktaalik. And Prof. Zhe-Xi Luo was part of a team of researchers from UChicago and Beijing who discovered fossils of the earliest-known tree-dwelling and ground-dwelling mammals.
How does evolution happen?
Evolution happens bit by bit. It’s all down to the fact that all organisms have very slightly different genetic codes—these are continually changing, by their parents swapping genetic material or just by random spontaneous mutations.
Any of these tiny changes can make a creature better or worse equipped to survive the world around them. For example, maybe it has slightly larger eyes that let it see better in the dark, or its skin has a unique pattern that means predators are a little less likely to find and eat it. It can pass those traits down to its children. Over time, these changes can add up to the point where a population is so different from a previous iteration that it could be called a new species.
What is human evolution?
Ever since Charles Darwin published his revolutionary Origin of Species in 1859, laying out the theory that all species evolve, people have been wrestling with the idea that humans are another creature that evolved—just like all the others we see around us.
Research teams are constantly finding new fossils of human ancestors (and side branches) from across the globe, but we can clearly see the distant ancestors of all modern humans and several other apes lived in Africa around 6 to 7 million years ago.
Researchers have found fossils that belong to a long human lineage represented by several species that they call “hominins.” These are species that belong to our evolutionary line after we separated from the our evolutionary cousins, the chimpanzees, around 7 million years ago. Humans and chimpanzees do not descend from each other, but share a common ancestor that is neither human nor chimpanzee.
In 1974, paleoanthropologist Donald Johanson, AM’70, PhD’74, discovered “Lucy,” a 3.2 million-year-old hominin skeleton. (She is nicknamed after the Beatles song “Lucy in the Sky with Diamonds,” which the researchers played often during the excavation.) Lucy’s skeleton was about 40% intact and her species, Australopithecus afarensis, may have been a direct ancestor of the modern human.
In 2000, UChicago Prof. Zeresenay “Zeray” Alemseged found the skeleton of a child of Lucy’s species who lived just a little before Lucy did—3.3 million years ago. This child, nicknamed “Selam,” which means “peace” in many Ethiopian languages, represents the earliest and most complete child skeleton in paleoanthropology. More complete than Lucy, Selam’s skeleton is over 60% preserved. “Selam has allowed us to address major evolutionary questions related to childhood, brain development, upright waling and climbing in her species, and gave us a unique window to study the youth of her time,” Alemseged said.
Another striking find was a set of fossil footprints of hominids walking on two legs from around 3.6 million years ago, which British paleoanthropologist Mary Leakey discovered and UChicago Prof. Russell Tuttle analyzed.
New discoveries continue to be made all the time, but evidence shows our ancestors are not only primates but also Tiktaalik and its cousins. In fact, we share common ancestors with every single life form on this planet, from fish to elephants, mushrooms to bacteria. It just depends how far back on the tree you look.
