Life is self-assembly and maintaining-order (or entropy) in space. Life is also memory-keeping and self-copy in time. Then how life is born on earth about 3.5 billion years ago? It is very possible that life started as a rare event, which generated both order of molecular structures and function of self-copy. So in a system which has molecules (like RNA or DNA) to memorize the body structure and self-copy function to pass order from generation to generation. ( It is the energy flow which maintains the entropy / order of life, please my an early blog谈谈生物物理（2）--- 生命违反热力学定律吗？), it will lead to entropy-decreasing and increasing of system complexity. If we reverse this process, the system’s entropy and complexity will change. So time-arrow is really impersonal and objective. There are two examples.

Crystal grows into single-handed structure

If entropy-maximum law is correct, it would be quite strange to observe that biological system on earth are all single-handed. From the perspective of either energy or entropy, left-hand and right-hand should occur equally in nature. Michael McBride etc. mentioned in their paper in Nature (Vol. 452, March 2008) that “ in a system with self-copying and destroying their mirror image, an initial random event in a tiny excess of one hand would lead to exclusive occurrence of that form, even if mirror-image version could also form randomly”. In crystal-growing process, with time evolves, single-handed structures dominate in the crystal and entropy decreases. So time arrow decides that we have a single-handed biological world.

Gene drifting

The similar process exists in life world as well. Supposing that we have several genotypes with equal fitness function in nature, after certain number of generation, it is surprising that usually one genotype will dominate the whole population even it has no obvious surviving advantages.

Conclusion

The biological evolution / decreasing of entropy is the emerging property of a dynamically changing network. So statistical physics and network dynamics will play an important role to understand life and evolution.

This week there is an international conference held in the institute about “biologically-inspired network”. Many big guys in the field of complex system come to give speeches. I would like report some ideas if I hear something interesting.

 

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