For a few years I've had this rather crazy idea in my head, I recently mentioned it to a couple people, but now thought it prudent to write about it here for posterity's sake (since the people I told it to probably didn't care much).
Most of us learn a little bit about genetics in grade school, about how the Y-chromosome is what determines whether a fertilized egg becomes a male or female, how brown eyes are a dominant gene and blue eyes are recessive—stuff like that.
But what I (and I suspect most people) didn't learn, was that genes aren't just passed vertically from parents to offspring (or parent in the case of asexual reproduction), but that there is also the possibility of horizontal gene transfer (also called lateral gene transfer). It occurs when genetic information from one organism finds “its” way into the genome of another organism by means other than being passed down by a parent during reproduction. It's a newer discovery, and it's hard to pin down because if it's passing between species with similar genomes, how would we even know it happened? Realistically the only way to know whether it's occurring between humans would be to sequence a large number of people's genomes repeatedly, and look for crossover. At the moment full sequencing of a person's genome remains non-trivial, and doing it repeatedly just makes it more difficult still. We now know it's actually the dominant form of evolution for prokaryotes, like bacteria, but I think we probably expect it to be less common in eukaryotes, after all, the cell nucleus should function to do just that: prevent foreign material from entering the genetic code. (Still, viral and bacterial infections destroy cells, and there's no physical law preventing them from injecting DNA into healthy cells.)
So the crazy idea is that maybe species that are in close proximity to one another occasionally share genetic code, and maybe some of the most unique traits about humans are shared with some of our domesticated animal friends with whom we've been sharing our habitat with for a very long time. The two cases that I've thought of are: the fact that dogs (apparently) tend to look at one side of our face over the other, (it seems that humans express more emotion on one side of their face than the other). Could it be that humans evolved this as we became an increasingly social species, and increasingly reliant on communication, and then when we began domesticating dogs tens of thousands of years ago the responsible gene(s) found their way from humans to dogs? (Through for example a shared pathogen, like a flu virus that infects both dogs & humans.)
The second one is horses and sweat. Most animals don't use sweat for thermoregulation of their bodies the way humans do, but horses are an exception to that general rule. Other primates sweat too—could it be that the close proximity of horses & humans over the last many millennia enabled the primate adaptation of using sweat to regulate body temperature over to horses? Another, probably much more mainstream hypothesis would be that it is convergent evolution, which is very common. Eyeballs, flight, camouflage—many adaptations are happened upon by completely disparate organisms for the obvious reason that if they lead to a survival benefit they are selected for, naturally.
Other crazy ideas would be that dark matter, inflation, and the Big Bang, are all incorrect. (I used to include dark energy in this, but now think might just be another indicator that our understanding of gravity is incomplete. Neil deGrasse Tyson was on The Colbert Report years ago and explained how we sort of messed up calling it dark matter, and that a better term would be dark gravity, since all we really know is that our theory of gravity doesn't match our observations (on the very large scale)—since we model dark energy as a cosmological constant inside general relativity it is already just a modification to our theory of gravity.)
Another crazy idea: could that "cold shiver down the spine" feeling we get during the big reveal of the villain in movies, or the crescendo in great music—could that be a remnant of the involuntary shaking mammals do to dry off?
I actually don't think this one is crazy: I suspect that the reason red light doesn't interfere with night vision is because our species has been around campfires & coals for so long our eyes have adapted to remaining in night vision mode even when bright light of long wavelengths (like hot coals) shined in them.
Okay this is probably the craziest idea I've put here yet but... what if there is a black hole at the center of the Earth & likely other planetary bodies, maybe even solar bodies. The idea is, the hawking radiation would push outward, while matter from the Earth would fall inward, in some kind of equilibrium. The part of Earth's core that appears to be solid would in fact be hollow, filled with this violent transfer of material into the black hole and energy out of it, giving it the appearance (in seismography) of being solid. (Actually that probably refutes this idea, because a hollow center would act differently, and I think we do get waves on seismographs traveling through the solid core.)
Some of the craziest ideas yet. Black holes emit radiation inversely proportional to their size, so the smallest blackholes actually release an incredible amount of radiation and then cease to exist. Got me wondering about the “equilibrium” point, where, the absorption would be equal to the emitted radiation. Found a quora answer about that for black holes absorbing cosmic background microwave radiation. But now imagine there were matter around the black hole, such that the black hole ate some of the matter and radiated it back out in an equilibrium. Imagine that was what lies at the center of our planet, and maybe all planets, and all stars. It could give the appearance of a solid core at the center, when in fact it is a hollow core.
And another crazy cosmology idea, though even more ambiguous still. Something about the expansion of space being related to matter. Imagine if instead of all of spacetime expanding, it was more that the places with matter are “sinking” away from one another. How to formalize this idea…
Or another similarly crazy idea. That instead of space time expanding throughout the universe, everywhere there is matter is just constantly “falling deeper” into spacetime, away from all other matter. Gravity would have to counter act this I suppose, keeping two sufficiently heavy and sufficiently close collections of matter from getting further apart. Should think some about to possibly formalize this idea with math.
Okay, for many years I've been vocally skeptical of the standard model of cosmology (Big Bang + general relativity + inflation + dark matter + dark energy). In high school I joked that dark matter and dark energy would probably cancel one another out (we had a screen saver that joked that there's enough sand in Northern Africa to fill the Sahara desert; in college I joked that sea level rise due to global warming might be just right to turn the Sahara into jungle; this is all a digression). I used to argue with Ethan Siegel that dark matter dark energy & inflation all felt like post hoc additions to Big Bang + general relativity to preserve them in light of the observations. I've seen grown to accept dark energy on the basis that it is easily incorporated into general relativity (and tonight I read that some early generalizations of general relativity even claimed the cosmological constant emerged naturally from the theory!). But as more and more searches for dark matter have come up empty I have remained skeptical of it. Sean Carroll gave a great lecture where he showed some bumps in the spectrum (of the CMB I think?) are most easily explained with dark matter, but I felt that same talk actually sounded really good for modified gravity scenarios, despite their apparent inability to explain those bumps. Anyway, read that tonight led me to the page on steady state theory which has sort of been the model in my head all this time. I had long though steady state only referred to models of a static universe, which were ruled out by Hubble's observation of redshift. But then reading the cosmological constant talk page (about the disputed claim that Einstein referred to the cosmological constant as his “biggest blunder”—sounds like he probably didn't say that, though may have referred to it as a mistake, but probably regretted telling FDR he should move forward with building the atomic bomb more than anything else in his life), the discussion there mentioned a constant creation cosmology, which led to me searching for that, which led to the steady state page.
A constant creation model is what has been floating around my head vaguely. I've long wondered if it could be connected to the virtual particle sea of quantum mechanics. In fact years ago I read a paper by Preskill (on his website) about black holes where he mentioned that in quantum mechanics it was important that there was no energy input into the universe, no wait, that isn't right. But some postulate of QM that is important because the introduction of a little violation leads to severe violations—and I wondered, that sounds an awfully lot like the sea of virtual particles! Something about a minimum energy I think. I've also wondered if this could all connect to Hawking radiation, as it seems that if particles were being created throughout the universe constantly, we could maybe have a state where the vast intergalactic spaces generate a soft trickle of particles which fall in to galaxies, and the expansion of spacetime due to the cosmological constant keeps it all in a steady state.
Apparently one of the biggest flaws with the steady state model—one of the final nails in its' coffin—was the cosmic microwave background radiation, which is consistent with the Big Bang, but not steady state models. But again, it feels like this could be the slow hum of particle creation.
So I keep excusing the refutations of the steady state model, but one really good criticism is that when we look around things that are further away do look different, as if the universe really is changing over time. I suppose the James Webb Space Telescope will help us with better observations of the most distant objects. But in order to defend a steady state model we'd need to explain why it is that radio sources seems to be only far away (and hence long ago), rather than also occurring nearby (and more recently).