The classic explanation is that elaborate and complex structures must exist because they confer some functional benefit to the organism, which is why natural selection drives ever greater states of complexity .
Clearly, in some cases the complexity is adaptive, such as the evolution of the eye (complex eyes see better than simple ones). But at the molecular level, a new study has found that there are other simple mechanisms that drive the accumulation of complexity.
The new study , conducted by researchers at the University of Chicago, suggests that elaborate protein structures accumulate over time, even when they serve no purpose, because a universal biochemical property and the genetic code force natural selection to keep them.
Most of the proteins in our cells form specific complexes with other proteins, a process called multimerization . Many proteins, especially those with high molecular weights, have a quaternary structure, this means that they are made up of several polypeptide chains (from two to hundreds of them). Each of these chains is called a subunit, and the union of several subunits is what we have called a multimer, or multisubunit protein.
Like other types of complexity in biology, multimers are often thought to persist through evolutionary time because they confer some functional benefit favored by natural selection.
To prove this, the study looked at the evolution of multimerization in a family of proteins called steroid hormone receptors , which assemble in pairs (called dimers). To do this, they used a technique called ancestral protein reconstruction, which allowed them to recreate ancient proteins in the laboratory and experimentally examine how they were affected by mutations that occurred hundreds of millions of years ago.
To their surprise, they discovered that ancient proteins did not function differently when assembled into a dimer than if they had never evolved to dimerize. There was nothing useful or beneficial in the formation of the complex . The explanation for why the dimeric form of the receptor has persisted for 450 million years turned out to be surprisingly simple, explains Georg Hochberg , one of the study’s authors:
These proteins gradually became addicted to their interaction, although there is nothing useful about them. The parts of the protein that form the interface where the partners bind accumulated mutations that were tolerable after dimer evolution, but would have been deleterious in the single state. This made the protein totally dependent on the dimeric form, and it could no longer go backwards. The useless complexity took hold, essentially forever.
The researchers therefore suggest that simple biochemical, genetic and evolutionary principles make the entrenchment of molecular complexes inevitable. This mechanism, operating on thousands of proteins over hundreds of millions of years, could drive the gradual accumulation of many useless complexes within cells .