What’s a “Missing Link”?

When Darwin published Origin of Species, one thing was missing from his argument: a “missing link.”

Though the term never once appears in the book, Darwin knew that his claims could benefit greatly from paleontological evidence of a species transition—an intermediate species connecting, for instance, humans to apes and monkeys. Less than two years after the publication of Origins, he got his wish. On January 3rd, 1863, Charles Darwin received a letter from his paleontologist friend Hugh Falconer with news of a tantalizing find: Archaeopteryx.

This extraordinary fossil—bearing feathers as well as teeth, claws, a bony tail and other reptilian traits—was just the sort of creature that Darwin’s theory of evolution by natural selection predicted should exist. The feathers left no question that the Jurassic Archaeopteryx was a bird, but the creature also had a suite of saurian traits that pointed to a reptilian ancestry.

Falconer could hardly contain his glee. “Had the Solenhofen quarries been commissioned—by august command—to turn out a strange being ‘a la Darwin,'” he wrote his friend, “it could not have executed the behest more handsomely—than in the Archæopteryx.”

Today, some still refer to Archaeopteryx as that long-sought “missing link” between birds and dinosaurs. It certainly checks a lot of boxes for an animal that seems between what were thought to be two distinct categories of organism. But there’s good reason not to use the phrase—which Darwin himself knew. As Nicholas Pyenson, Smithsonian National Museum of Natural History curator of fossil marine mammals, puts it: “Life is really a tree, not a chain.”

“To me, the idea of a ‘missing link’ implies a linear chain of one species evolving into another, evolving into another, and so on,” says Smithsonian Human Origins Program anthropologist Briana Pobiner. That isn’t the pattern we see. Instead, evolution “produces a tree-like branching pattern with multiple descendants of an ancestor species existing at the same time, and sometimes even alongside that ancestor species.”

The chain metaphor that “missing link” implies would have us looking for straight lines, when the reality of evolution is much more discursive. Not every fossil creature can be slotted in as a direct ancestor to something alive today. That’s why paleontologists have come to abhor the term: it obscures the true pattern of evolutionary change.

By Any Other Name

But what to call “strange beings ‘a la Darwin' like Archaeopteryx, whales with legs, and humans that look like apes?

Paleontologists often prefer the term “transitional form” or “intermediate form,” because they imply that these species are parts of an ever-changing continuum. This isn’t just a matter of splitting hairs; erminology shapes our ideas and the way dramatic changes in the course of life are interpreted. Before (and even after) Darwin, naturalists sometimes saw species as part of a ranked hierarchy in which newer forms were somehow better than what came before. “Sloppy words lead to sloppy thinking,” as Pyenson says.

“In some sense, every species in a transitional form from its ancestor because it retains many ancestral traits but has enough unique traits to be a separate species,” Pobiner says. And given that every species alive today has fossils related to its ancestry, that’s a lot of transitional fossils. More often, Pobiner says, “paleontologists often use this term when talking about larger anatomical or ecological shifts that occurred during the history of life.”

Not that "transitional form" is without its own problems. The phrase can sometimes inadvertently cast an evolutionary cousin as an ancestor through popular translation. But it at least highlights that the organism in question helps inform what paleontologists have identified as a major shift in life’s history.

Evolution is constantly branching out, and drawing out lines of descent—from one ancestor species to its direct descendant—is almost always impossible due to the incomplete nature of the fossil record. “I look at the natural geological record,” Darwin wrote, “as a history of the world imperfectly kept.” Relating strata to pages of a book, he continued: “Of this volume only here and there a short chapter has been preserved; and of each page, only here and there a few lines.”

Paleontologists know these lines well, for of all the life that ever existed only a fraction was preserved and an even smaller portion as yet found. What's truly amazing, then, is that we are able to detect major changes at all!

How The Whale Got Its Handbones

We know much of the remarkable evolutionary story of whales thanks to transitional fossils. The very first whales, for instance, didn’t look anything like the minkes and orcas swimming around today’s oceans. About 55 million years ago, they were terrestrial animals with hooved feet that looked something like small deer with long tails. They were artiodactyls, members of the same group of mammals that includes hippos and cows today.

Over the course of about 10 million years, early whales at the water’s edge became increasingly amphibious until only the totally aquatic forms were left. This required major changes to how whales moved, what they ate and their senses. A growing accumulation of fossils since the 1970s inform how these changes unfolded; at the same time, you can see whales’ past forms in telltale signs like the handbones in a blue whale’s fin.

An entire flotilla of early whale fossils outlines these changes, such as feet that became paddle-like, spines adapted to up-and-down undulation to swim, and teeth suited to nabbing slippery fish. “Whales don’t look anything like their nearest relatives” alive today, says Pyenson, who is the author of the upcoming book Spying on Whales: The Past, Present, And Future of Earth's Most Awesome Creatures. "Fossils are what tells us about these connections.”

This is why the fossil record is so essential. “If we only had DNA to go on and no fossil record,” Pyenson says, “we’d still be scratching our head as to where whales came from.” 

The Human Transition

Whales aren’t unique, of course. Transcendent evolutionary change applies to every organism, from redwoods to whales, from dinosaurs to sea slugs—to us. In fact, we are one of the core problems with the phrase “missing link.”

Many people associate the phrase distinctly with humans. For them, it conjures up the image of a beetle-browed, half-human, half-ape creature that would slot right in between us and chimpanzees. But, as we know, evolution doesn’t proceed along a linear path that would spit out such a being: we have a family tree, not a family ladder. Instead of a single fossil that answers all our questions, what we have is a varied group of fossil humans who help us understand that we are just part of a much larger tale.

There’s also a political reason experts have often avoided using the term. Anti-evolution organizations like Answers in Genesis and the Discovery Institute have often claimed that “missing links” are exactly that: missing. For every new facet of evolution a particular organism might show us, there is an evolution denier pointing to what has not yet been found as if it’s disproof. Relying on the term “missing link,” in other words, gives to much advantage to anti-science agitators, giving scientists all the more reason to abandon the term.

In reality, the human story stretches back millions of years, leaving us the last species standing—literally. We habitually walk in a way that no other animal does with our backs totally upright and our legs beneath us. How that happened has been a major research focus as paleontologists and anthropologists have looked to our past.

This change occurred relatively early on, between the time our ancestors split from chimpanzee ancestors over 6 million years ago and about 3.6 million years ago when prehistoric people walked through ash and gave us definitive proof that early humans were walking much like we do. But the story of humanity goes beyond the legs and spine. “The earliest hominins also had relatively smaller canines than other apes do,” Pobiner says, one of many changes related to alterations in diet, behavior, and more.

We've been able to piece together many of these changes thanks to transitional fossils. Without a modified DeLorean or TARDIS, though, we are left with the imperfect, incomplete, and nonetheless illuminating fossil record: an epic story of life in transition.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.