Leftist bias may have a chemical reason
For more than a century, scientists have wondered why the 20 amino acids used by all living things are exclusively left-handed - now, U.S. chemists believe they have the answer.
The 'left-handedness' seen in the basic building blocks of all living things - amino acids - has ancient origins, said the team, whose work appears in the German chemistry journal, Angewandte Chemie.
Simply put, if you weren't a 'leftie' in the primordial chemical soup where amino acids were born, and which went on to give rise to life billions of years ago, you had no future, the team argue. Amino acids are the carbon-based molecules that make up proteins and enzymes, essential for the survival and reproduction of all life.
One of the authors of the paper, Professor Graham Cooks of Purdue University in Indiana likened the soup of naturally-occurring amino acids - the building blocks of proteins - to a dance nightclub, in which partnering was essential to create life.
In Cooks view, a particular 'left-handed' amino acid, called serine, acted as a 'nightclub bouncer' - and one with attitude. "If you weren't a left-handed amino acid, you couldn't get in to partner up and dance," said the analytical chemist.
The work focuses on the left-handed asymmetry, or 'chirality', found in all 20 amino acids used by all living things on Earth. The property refers to the direction the molecules twist. While their chemical properties are generally the same, the dominance of left-handed chemicals over right-handed ones in nature has mystified scientists for over a century.
While most amino acids form weak bonds with each other regardless of their chirality, serine is unusually selective: it forms tightly-bound clusters of eight molecules - either all right-handed or all left, with no mixing, Cooks said.
His group's experiments suggest serine could have "set the left standard" aeons ago, by forming left-handed clusters with strong bonds, able to link only to other left-handed amino acids: "We believe serine was the first biological molecule to make a chiral choice, possibly one of the root steps in chemical evolution itself.
"Because of serine's ability to form these strong bonds, it essentially forced all other amino acids to twist its way. Once the serine took the left fork in the road, only the 'lefties' in the primordial soup got to partner up for the dance of life," he said.
But why the left path and not the right? It could be simply chance. "We're not sure whether left-handed serine's dominance in the earliest period of life's prehistory was arbitrary," Cooks said.
"But serine can switch its chirality under mild conditions. If somehow polarised light, for example, or a swirling motion in water were present at a critical moment, some of the right-handed clusters could have become left-handed. This could have cascaded into other prebiotic reactions and set the pace for a billion years of evolution."