Do you have any evidence that chemotrophs do not have an isotopic preference? That would contradict the &quot;all life&quot; from the Wikipedia quote, and contradict what little I know about the universality of the mechanism. A quick search finds:<p>Detmers, J., Brüchert, V. Habicht, K.S. and Kuever, J. (2001) Diversity of Sulfur Isotope Fractionations by Sulfate-Reducing Prokaryotes, Applied and Environmental Microbiology, 67, 888-894. <a href="http:&#x2F;&#x2F;www.ncbi.nlm.nih.gov&#x2F;pmc&#x2F;articles&#x2F;PMC92663&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.ncbi.nlm.nih.gov&#x2F;pmc&#x2F;articles&#x2F;PMC92663&#x2F;</a><p>&gt; All of the 32 sulfate-reducing bacteria discriminated against 34S during sulfate reduction. Desulfonema magnum showed the largest fractionation (ɛ = 42.0‰), and Desulfovibrio halophilus showed the smallest (ɛ = 2.0‰)<p>It looks like the scientists working in this field have not made the assumption but have actually tested it.<p>&gt; This is why you only see chemotrophs around local high concentrations<p>I lack the biochemistry knowledge. My limited understanding is that life was restricted to the ocean. If the Fe and O react on land (with limited oxygen, that produces iron(II), yes?), then it will be dissolved in fresh water. But when it hits the ocean, it&#x27;s going to interact with the salts and other substances which a billion years of rain bought into the sea.<p>In that case, the ocean edge will have the local high chemical concentrations. Sure, over time that will equilibrate. But any life at that boundary will be able to take advantage of the chemical gradient in the way you mentioned. We see this now, where continental runoff is a source of nutrients for ocean life, similar to how volcanic eruptions can add nutrients to soil through the weathering process of volcanic rocks.