"Tiktaalik" & circadian rhythm evolution in humans
June 20, 2020 2:14 AM   Subscribe

Infants don't have a circadian rhythm. When did humans evolve a circadian rhythm? Did the Tiktaalik have a circadian rhythm?
posted by jago25_98 to Science & Nature (7 answers total)
Apparently all animals have circadian rhythms.
posted by Segundus at 4:25 AM on June 20, 2020

Not sure where you are getting the idea that infants don't have a circadian rhythm. Babies develop it in the first few months of life.
posted by Megami at 5:50 AM on June 20, 2020

Even bacteria have circadian rhythms, although theirs evolved along a different pathway than eukaryotes (plants, animals, fungi, etc.). And apparently many animals that live in non-rhythmic environments still have circadian clocks, according to this interesting and reasonably accessible review article.

It’s probably better to think of newborns’ relative lack of circadian behavior compared to adults as being like their lack of hair. Although it was once popular to say that development parallels evolution (“ontogeny recapitulates phylogeny”), that’s mostly not true.
posted by mskyle at 5:58 AM on June 20, 2020 [6 favorites]

Newborns may not have a circadian rhythm since it takes them ~60 days to ramp up melatonin production, but fetuses do. Sharing hormones allows the mother and fetus to synchronize to a daily rhythm.
posted by ananci at 7:22 AM on June 20, 2020 [1 favorite]

Infants have a very predictable ultradian rhythm (cycle < 24 hr) which is further divided into quiet and active sleep. There is a whole scoring manual for infant sleep patterns; scroll down to Table 4 for a discussion of what their super-short sleep cycles look like. These are present from 37 weeks conceptional age (i.e. at term; probably develop sometime before then, but fetal EEG is not really A Thing.) Then scroll farther to Table 6 for a gander on how sleep cycles change over childhood/adolescence.

tl;dr: Human sleep is not really "circadian" in the sense of a 24-hr day; we just use that as a shorthand for the fact that we mostly reset our sleep patterns daily with sunlight and pesky things like clocks. Neonates are new to the whole idea of "sunlight" and couldn't care less about clocks (as new parents can probably attest).
posted by basalganglia at 9:34 AM on June 20, 2020 [2 favorites]

My babies developed what seemed to be pretty regular old 24 hour circadian rhythms within a few days of birth. Sure they're confused when they're first exposed to sunlight and sure when their tummies are tiny they wake up a few times a night to eat, but it's clear from their sleep patterns that their bodies innately figure out that there's a time of day when they're awake more and a time of day when they're asleep more, especially if you're pretty disciplined about keeping the blinds cracked in their bedrooms until they aren't day/night confused anymore.
posted by potrzebie at 11:16 AM on June 20, 2020

To try and focus on the "why did humans (as well as other organisms) evolve a circadian rhythm?"

The evolutionary reasons humans have (as opposed to "evolved") circadian is pretty obvious. Humans are eyesight reliant and most effective during the day. It makes a lot of sense to stop moving at night--we conserve energy, are harder for predators to find, etc. If you throw in that we *need* some downtime it obviously makes a lot of sense for us for it to happen at night. You want to be sleepy when it's time to sleep. Keying some biochemical cycles to day/night cycles is just evolution doing it's thing. Other animals have similar preferences--whether nocturnal, diurnal, crepuscular, etc. Note there are a ton of other things organisms can do by keeping track of day/night cycles, like if you're a bird you can know that fall is beginning (even if it's unseasonably warm) and time to migrate.

I say this isn't why we "evolved" the cycle because it appears the cycle was already there. One paper I just skimmed listed a few hypotheses that imply this is useful way, way back: for example, a single celled organism might have really wanted to adjust it's biochemical processes to protect itself when a neighbor was emitting toxic oxygen from photosynthesis. (Incidentally the biochemistry is really well worked out and a Nobel prize was awarded a few years ago.)

So my way of looking at is that as humans we already had this machinery to keep track of time and the 24-hour clock, and due to natural selection we didn't discard it (because it was useful) and we started attaching these other processes, useful to our multicellular selves, to the same internal clock as well.

Finally, Tiktaalik certainly had a Circadian rhythm because plants, fungi, bacteria and animals all have it. However, it appears to have evolved differently across those kingdoms so it's possible they had a different mechanism. (I tried to see if the cnidarians, as some of our most distant living metazoan relatives, shared our pathways and didn't really get a clear answer, but seems to be "mostly no", FWIW)

(I should do disclaimer that I'm very much a layman here.)
posted by mark k at 8:56 PM on June 20, 2020 [1 favorite]

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