Is there any evidence or example of "trade" or "money" in single cell organisms? Single cells (like bacteria or eukaryotes) are extremely complex "machines" which can take input and produce output, and generally just _live_ life. The question is, is there any example of "trade" (as in, "commerce") between cells, or within a cell at the molecular level (trading one protein for another, or a generalized "currency" where you could trade a general thing like money for anything else? I imagine there are, but not sure if people have put cells into this mental framework before. Receptors on a cell seem to be somewhat a form of exchange/trade, "give me a molecule and I will start this chain reaction for you". There's probably many other forms of trade in this light. But what about money/currency? This is, generally, an open-ended question. To make it closed. Provide one complete example of trade and money, and/or link to relevant research.

Lichens might be such an example. Algae living with fungi in a mutualistic relationship.