Читать книгу Astrobiology - Charles S. Cockell - Страница 45

Although terrestrial biochemistry makes extensive use of metal ions, such as iron and copper as cofactors in enzymes, life does not fabricate metal structures. This is surprising given the vast use of metals and metallic structures in human engineering. Why aren't bones made from steel for example, or horns in animals made from titanium? There are a number of possible reasons that may not be mutually exclusive: (i) It may be energetically too expensive. Most metals are not pure in the environment and considerable energy is used in human manufacturing processes to purify them. Locked up in sulfides, carbonates, and other compounds, they are difficult to purify biologically. (ii) Pure metals have large densities. For example, compare the density of steels (typically ∼8000 kg m⁻³) to calcium phosphate bone (typically ∼3800 kg m⁻³) and one can see that metal structures would come with a considerable mass burden. (iii) Metals are generally at low abundance in the environment. It should be pointed out that this argument holds for some metals such as vanadium, but it clearly does not for metals such as iron and aluminum, which constitute 8.1% and 5.0% of Earth's crust, respectively. The preceding three arguments assume that nature could have attempted to use metals but for various reasons rejected them. However, we have no evidence of extensive experiments in metal construction in biology. It could be that pathways to other high-strength compounds are evolutionarily simpler than pathways that involve the extraction and purification of metals from compounds naturally found in the environment. What do you think? Would alien life be any different?