The idea that a scientific theory can be ‘falsified’ is a myth
J.B.S. Haldane, one of the founders of modern evolutionary biology theory, was reportedly asked what it would take for him to lose faith in the theory of evolution and is said to have replied, “Fossil rabbits in the Precambrian.”
Since the so-called “Cambrian explosion” of 500 million years ago marks the earliest appearance in the fossil record of complex animals, finding mammal fossils that predate them would falsify the theory.
But would it really?
The Haldane story, though apocryphal, is one of many in the scientific folklore that suggest that falsification is the defining characteristic of science.
As expressed by astrophysicist Mario Livio in his book Brilliant Blunders:
"[E]ver since the seminal work of philosopher of science Karl Popper, for a scientific theory to be worthy of its name, it has to be falsifiable by experiments or observations. This requirement has become the foundation of the ‘scientific method.’”
But the field known as science studies (comprising the history, philosophy and sociology of science) has shown that falsification cannot work, even in principle.
This is because an experimental result is not a simple fact obtained directly from nature. Identifying and dating Haldane's bone involves using many other theories from diverse fields, including physics, chemistry, and geology. Similarly, a theoretical prediction is never the product of a single theory, but also requires using many other theories.
When a “theoretical” prediction disagrees with “experimental” data, what this tells us is that that there is a disagreement between two sets of theories, so we cannot say that any particular theory is falsified.