Evolution skeptics argue that some biological structures, like the brain or the eye, are simply too complex for natural selection to explain. Biologists have proposed various ways that so-called 'irreducibly complex' structures could emerge incrementally（递增地） over time, bit by bit. But a new study proposes an alternative route. Instead of starting from simpler precursors and becoming more intricate（复杂的）, say authors Dan McShea and Wim Hordijk, some structures could have evolved from complex beginnings that gradually grew simpler -- an idea they dub "complexity by subtraction." Computer models and trends in skull evolution back them up, the researchers show in a study published this week in the journal Evolutionary Biology.

Some biological structures are too dizzyingly complex to have emerged stepwise（逐步地） by adding one part and then the next over time, intelligent design advocates say. Consider the human eye, or the cascade that causes blood to clot, or the flagellum（鞭毛）, the tiny appendage that enables some bacteria to get around. Such all-or-none structures, the argument goes, need all their parts in order to function. Alter or take away any one piece, and the whole system stops working. In other words, what good is two thirds of an eye, or half of a flagellum?

For the majority of scientists, the standard response is to point to simpler versions of supposedly 'irreducibly complex' structures that exist in nature today, such as cup eyes in flatworms. Others show how such structures could have evolved incrementally over millions of years from simpler precursors. A simple eye-like structure -- say, a patch of light-sensitive cells on the surface of the skin -- could evolve into a camera-like eye like what we humans and many other animals have today, biologists say.

"Even a very simple eye with a small number of parts would work a little. It would be able to detect shadows, or where light is coming from," said co-author Dan McShea of Duke University.

In a new study, McShea and co-author Wim Hordijk propose an alternative route. Instead of emerging by gradually and incrementally adding new genes, cells, tissues or organs over time, what if some so-called 'irreducibly complex' structures came to be by gradually losing parts, becoming simpler and more streamlined? Think of naturally occurring rock arches, which start as cliffs or piles of stone and form when bits of stone are weathered away. They call the principle 'complexity by subtraction.'

"Instead of building up bit by bit from simple to complex, you start complex and then winnow out the unnecessary parts, refining them and making them more efficient as you go," McShea said.