Subtitles section Play video Print subtitles Why do we have one of some things, and two of other things? The logistics alone are enough to cripple even a pro like me! Happy Monday DNews lovers, thanks for watching! Y'all really are the best. Every Monday we answer one of YOUR questions from the comments and this week, someone named "Dern Sure" wants to know why we have two of certain organs, like, Kidneys, lungs, testes, but not multiple hearts, livers or brains? Scientists debate the answer to this, but it really comes down to how we were locked into our evolutionary path. 550 million years ago, the acoel was the first bilateral organism. Bilateralism is the tendency of some lifeforms to be symmetrical. Today, many organisms -- especially vertebrates -- have inherited bilateral symmetry. If you drew a line down the middle of most primates, birds, fish, crustaceans, amphibians, and MANY others… you'd see a mirror image (or close to it). Though we don't know EXACTLY why, there are definitely some hypotheses. Like, scientists speculate a bilateral body can move more easily because the appendages are mirrored, or perhaps it's easier to support a central nervous system with a central spinal cord. It's just one of those things we don't know. But, I mean, we've held onto the concept for more than 500 million years, so there must be a good reason for its evolution. 300 million years ago, some animals moved from the sea to the land, toting their evolutionary advantages with them. Which might explain why most land animals are bilaterally symmetrical. But let's not get specist. Undersea animals might have continued to evolve, eschewing bilateralism. Starfish and many undersea animals developed radial symmetry, and others, like sponges, aren't symmetrical at all! So, it's not entirely universal! I know that's not the answer YET, but we're getting there. Evolutionary anthropologists guess that we have two limbs, two lungs and two kidneys because we needed them for some reason! They're NOT spare parts, but instead, the dual organs gave ancestral organisms an advantage in some way. For example, you can SEE with one eye and sure enough, one eye evolved first, but two eyes are required for depth perception. Organisms who developed bilateral eyes were better able to focus on prey; or to see the predators coming. But what about kidneys? A study in Nephrology, Dialysis, Transplantation looked at how people did after getting a kidney removed, and surprisingly found they did better for around 16 years, but like, 24, years later, had declines in renal function. They're not spares, but we still live if they're damaged. We just won’t be at 100%, and the brain does too as well as a number of our other organs. So, why one heart and two of a lot of other stuff? Well, humans start as an egg, and begin to divide. At some point in our embryonic stage, we're literally a blob with a tube down the middle that becomes the digestive system. Along that center line the vital organs (liver, lungs, heart, pancreas) begin as tiny buds. The liver and brain each form from single buds; the lungs, however, come from two, just like the kidneys, which develop buds on complete opposite sides of the centerline. Interestingly, the heart is right in the middle… and according to the book, "Heart Development, Volume 342," amphibian bilateral hearts form as two bilaterally symmetrical buds next to each other which eventually merge into one heart! But, if they're artificially interrupted, they'll form two independent hearts! It would be unethical to do this with humans, of course, time lords on the other hand……… Okay, so… if we take into account symmetry and evolutionary advantage. It would seem to me, having two brains would be more trouble than it's worth, considering how resource heavy they are, right? We have two of some things because we've been symmetrical for over 500 million years, and though we COULD have two hearts, we simply, don't. I hope we enlightened you, Dern Sure. Like I said, we answer viewer questions all the time and we might’ve answered yours in the past. Questions like, Am I crazy if I talk to myself?