Origin of life: cooperative evolution of different types of molecules could have led to self-reproduction - a major step

Life on Earth may have come to existence as a result of cooperative interactions between molecules of different types. This is a conclusion of the computer simulation study recently published in the current issue of the Origins of Life and Evolution of Biospheres. This work suggests a possible answer to one of the most difficult problems in studying origin life: a very low probability of spontaneous formation of functional life molecules capable of self-reproduction.

The author, Maya Fishkis, was driven by a quest to find a plausible behavior of nonliving molecules that could lead to formation of the earliest, simplest unit we can call living. Such a unit is usually called a protocell, a predecessor of a cell.

The simplest molecules that constitute living organisms are common in nature. Our Earth has been formed about 4.5 billion years ago from a protoplanetary disk. Astronomers observed such protoplanetary disks in many locations in the universe. Using radio telescopes they found surprisingly complex organic molecules attached to the dust particles in these disks. The same molecules have been found in celestial bodies that still collide with Earth, such as meteorites, comets, and meteoritic dust. Among them, they found amino acids, building blocks of proteins, and nucleobases, parts of nucleotides which are building blocks of nucleic acids, genetic molecules. These molecules constitute a cosmic source of the materials needed for the origin of life. The other source could have been synthesis on the Earth from atmospheric gases and water, as has been originally shown by Urey and Miller in 1953 via electric discharge and later extended to a variety of other possible energy sources.

In order to form a living organism of the type that we have on Earth, these molecules have to polymerize into at least 2 types of functional long polymers. One is enzymes or catalysts, molecules that accelerate chemical reactions involved in life processes; the other is carriers of genetic information. Most of the enzymes are protein molecules, but there is also increasing evidence that RNA molecules serve as enzymes in a variety of important chemical reactions in living organisms. The carriers of genetic information are RNA and DNA molecules, also called nucleic acids. Both types of molecules - enzymes and nucleic acids - consist of long chains of monomers, arranged in a particular sequence. The major difficulty in devising how life could have originated is the very low probability of spontaneous formation of such molecules. How could the molecules of particular compositions (i.e. sequences of monomers) be selected out of the messy environment?

The approach taken in this work is that these molecules were formed in the process of chemical evolution that preceded biological evolution. If monomers of proteins and RNA molecules had been present in the environment containing a mineral surface that was periodically warm and dry, then cool and wet, for example, on the shore of some tidal lagoon, these molecules could polymerize into peptides (short proteins) and oligonucleotides (short RNA molecules). There is experimental evidence that such polymerization takes place on mineral surfaces in these environmental conditions.

It was also shown experimentally that even short oligonucleotide molecules can exhibit template properties whereas short peptides having particular compositions can work as catalysts. Although these template and catalytic properties were not as high as in long polynucleotide and protein molecules, they could be a source of mutual selection during a period of chemical evolution.
The mechanism of mutual selection could work as follows: the small RNA template contained directions for making a single protein enzyme which in turn helped the RNA template molecule replicate. The particular sequences of building blocks, nucleotides, in RNA molecules promoted formation of a matching sequences of amino acids in peptides. Some of these peptides exhibited catalytic properties. Due to the short molecular length, the probability of forming peptides of particular composition having catalytic properties was reasonably high. If these newly formed catalytic peptides attached themselves to the mineral surface in the vicinity of the oligonucleotides that coded for them, they could accelerate formation of these oligonucleotides. Then more of such oligonucleotide templates were created, which in turn led to the formation of more catalytic peptides and so on. Such cooperative evolution would lead to a higher rate of reproduction of the catalytic peptides and oligonucleotides that coded for them relative to the molecules of other compositions.

During a wet part of the environmental cycle, some of the molecules, mostly the smaller ones, reacted with water or were dissolved in water and left the mineral surface. Computer simulations have shown that gradually molecules of peptides having catalytic properties and oligonucleotides that coded for them reproduced themselves, accumulated and became the predominant molecules on a mineral surface at the expense of the molecules of other compositions.

The necessary conditions for the described mechanism was adsorption of the newly formed catalytic peptides in the proximity of the templates that created them and a finite rate of diffusion on a mineral surface. Reproduction errors were relatively high for such short molecules creating variability necessary for further evolution leading to the formation of longer genetic molecules and enzymes.

How could the self-replicating molecular units develop into protocells?

Based on the scientific data we have today, the following possible steps can be suggested.

Some of the short peptide molecules formed on the mineral surface and some molecules in the environment could have properties that made them capable of forming cell membranes.

During dry periods, the membrane-forming molecules could form layers of film on a mineral surface with the self-reproducing molecules trapped between film layers. Then, during wet periods, this film could swell and form enclosures containing the entrapped self-reproducing molecules. Experimental studies have demonstrated that such entrapment of large molecules is possible and that the molecules inside the membrane remain active.

These original membranes didn't form a perfect barrier. They had some pores that allowed relatively small molecules such as amino acids and nucleotides to pass from outside into the protocells. These molecules could serve as "food" for reactions leading to continuing assembly of peptides and small RNA molecules too large to leave the protocells through the pores. In addition, the membrane-forming molecules could have been formed inside the protocell and incorporated into the membrane causing membrane growth. Increased membrane tension would lead to division of the protocell into two smaller ones. At this point, natural selection would favour the protocells containing longer and faster reproducing molecules with better functional properties.

More information:
Fishkis M (2011) Emergence of self-reproduction in cooperative chemical evolution of prebiological molecules. Orig life Evol Biosph 41:261-275. www.springerlink.com/content/k893688773377265/

Provided by Maya Fishkis