Why are we born with 2 kidneys if we only technically need one to survive?
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It’s nice to have a back up. Some of our more smaller body parts have a redundancy in case of damage (eyes, testicles, ovaries, lungs, nostrils, ears, etc.). If one kidney fails, you don’t instantly die.
This might not work with something like a heart because that could be too complex/require too much food to maintain, to a point where it become a detriment. Instead, the heart is protect by the strongest part of the rib cage.
EDIT: Yes, I know that having two eyes, and two ears does more than simply provide a redundancy, but the redundancy element certainly helps).
Because kidneys get damaged pretty often. If you eat something poisonous or get into a fight and get punched in the kidney, you will damage your only kidney. With two one can work overtime while the other heals. With only one these senarios become potentially lethal.
Kidneys are easily damaged organs due to their location in the back therefore natural selection favored populations with two kidneys instead of one. This is actually a common thing in anatomy for example humans have two eyes despite needing only one for sight, two lungs despite still being able to breathe with one, two testes, two ovaries, two halves of the brain etc.
The short answer is that we were created by evolution, not intelligent design. As others mentioned, kidneys can be easily damaged and thus our ancestors who had two were probably more likely to survive to procreate. Alternatively, any mutation that caused our ancestors to have only one kidney wasn’t better at helping them survive to procreate, so it never won out.
I was only born with one, was 35 when I found out. No negative impact so far but kidney stones could cause a serious issue. My dad is the same, he was 55 when he found out. He started dialysis when he was late 70s. Not sure having two would make a difference though as we’ve always had just one so our 100 kidney has always done the work of 2.
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