Origin of life
Reassembling life’s starter kit
Dieter Braun wants to know how the first unicellular organisms evolved from prebiotic molecules. To find out he mimicks in experiment the environmental conditions that prevailed on the young Earth.
Dieter Braun has put the instructions for use of his lab’s espresso machine on his group’s website. So people from nearby labs know how to operate the apparatus (which is only a little less complicated than the instrumentation they normally work with). Chemists and physicists congregate around the machine to discuss ideas – having to raise their voices once the coffee grinder gets going!
Braun is not a particularly big fan of coffee, and he admits that he intended the machine to act as a lure. He wanted to create a corner in which people with different backgrounds and different interests could exchange views and ideas, because such interactions are the lifeblood of his work. For his area of interest spans the fields of Physics, Chemistry and Biology. Braun himself, a physics professor at LMU, refers to this hybrid discipline as ‘Systems Biophysics’, because, he says “we study the physics of biological systems”, including the ‘prebiotic’ systems in which life on Earth originated.
The increase in complexity from single-celled organisms to multicellular forms like green plants, fishes, sea-gulls, cattle, lions and primates can be understood in terms of the mechanisms of organismal evolution. But how molecules with the capacity for evolution first emerged, and what kind of protobiological system gave rise to the first true cell, remains shrouded in mystery.
How it all got started
There are, of course, hypotheses. Most researchers believe that the ancestral forms of today’s biopolymers were assembled from precursors that were readily available on Earth some four billion years ago. In other words, biological evolution was preceded by a phase of chemical evolution, during which ever larger and more complex molecules were produced.
This process probably began with the formation of ribonucleic acid or RNA, which is chemically close to deoxyribonucleic acid, DNA. Both are linear polymers, made up of varying combinations of four different types of ‘nucleotide’ subunits, which can pair up in defined ways. This allows single-stranded RNA to fold into specific shapes, and DNA to adopt the iconic double-stranded form. DNA is the more stable, and in modern cells it stores the genetic information, encoded in sequences of nucleotides. Copied into RNA, these sequences program the synthesis of proteins, which are made up of specific combinations of 20 different amino acids – and fold into specific conformations that endow them with the diverse biochemical capabilities required for cell function. RNA readily reacts with other molecules, and with itself – making it an ideal basis for chemical evolution.
It is not entirely clear how the precursors for the first nucleic acids and proteins were synthesized. But the more difficult question is how polymeric molecules capable of storing information, catalyzing reactions or performing specific tasks were assembled from such building blocks. In all model systems so far studied, short molecules predominate over long ones, simply because the former can be replicated faster. This essentially precludes further chemical evolution. But some years ago, Dieter Braun stumbled on a mechanism that favors polymer growth, and enables long molecules to be replicate and become more complex.
Avoiding the trap of equilibrium
A chance observation made more than a decade ago led to the crucial insight. At that time, Braun was a member of Albert Libchaber’s group at Rockefeller University in New York, and was investigating particle dynamics in thermal gradients. He observed the behavior of aqueous suspensions of silica particles in a reaction vessel which was locally heated with a laser beam. In this set-up, the particle notions reflect the action of currents set up by temperature differences in the suspension. “Under the microscope, you can see the particles whizzing about,” says Braun. But then he noticed that they began to accumulate in one area of the vessel …
Moreover, to his surprise, the larger the particles, the more efficient the trapping effect became. Braun realized that this phenomenon could solve the problem that all models of prebiotic evolution of life had run into. Thanks to this “thermophoretic” effect, molecules could be concentrated in a localized region instead of being distributed equally throughout the medium by diffusion. For example the oxygen concentration in an enclosed space at a uniform temperature is the same everywhere. Similarly, the precursors of biological polymers – the amino acids and nucleotides – would have been equally distributed in the primal ocean. “But to get life going, the system must be kept out of this most probable, equilibrium state,” says Braun.
“And temperature differences provide a means of doing this, thus enabling evolution to get underway.” Constrained and concentrated in a thermal trap, precursor molecules could have been linked to each other to form short chains, which would continue to grow as long as precursors were available. And at some stage, they became long and complex enough to direct their own replication, thus acquiring the ability to reproduce themselves – a crucial prerequisite for the evolution of living systems. By this means, the first informational molecules could have evolved on the young Earth, in the confined spaces within porous rock, for instance.