Why do bug bites and cuts itch when scratching them clearly doesn't help us survive? Did our early ancestors need to rely on this itching sensation, or is it just that the receptors for itches just happen to be stimulated by histamine?
Scratching is getting potentially poisonous or at least irritant substances out and removes lodged insects like ticks.
Removing subcutane parasites like scapies mites is not feasible without scratching yourself bloody.
I don't see how "scratching them clearly doesn't help us survive". Behaviors and skin reactions evolved way before our modern hygiene and parasite cures.
While itching and pain are related, research indicates that these sensations are caused by distinct nociceptors. Essentially, wounds itch because histamine is released as part of the wound healing process, and histamine activates those itch nociceptors. So perhaps the more apt question is just "why does histamine cause itching?" (See also answer to What is itching?)
As of now, we might only be able to speculate.
The following are a few hypotheses:
- Selecting for more specific itch nociceptors, which would incorporate some mechanism to prevent itching of wounds specifically, doesn't help influence survival enough to be useful, or would impose a cost more severe than the benefit.
- Itching wounds may have caused ancestral species to remove contaminants, IE by licking them.
- It could be a vestigial adaptation, a simple signal saying to the original "something happened here, do something about it!"
- New insights into the mechanisms of itch: are pain and itch controlled by distinct mechanisms?
- The accelerating effect of histamine on the cutaneous wound-healing process through the action of basic fibroblast growth factor.
- Physiology and healing dynamics of chronic cutaneous wounds.
- Histamine-induced itch and its relationship with pain
- Histatins are the major wound-closure stimulating factors in human saliva as identified in a cell culture assay.
Some insects produce chemicals to induce itching in order to further spread eggs laid in the wound. Some micro-organisms do the same, a cut may be infected.
It's perfectly possible that the urge to scratch is not beneficial in all situations but simply a majority of them. This would be more than enough reason to have the impulse stick around, evolutionary speaking.
The body does not differentiate very often between externally and internally produced chemicals.
For example, I am diabetic and take insulin, a hormone related to blood sugar. It is introduced to my blood stream from without, but my body reacts to it as if my pancreas had produced it, which is what happens in healthy individuals.
In an organic system which coordinates internally via chemical signals (hormones), it is impractical for parts of the system to attempt to screen out externally introduced chemicals, such as from a bug bite.