Kidneys are a sign of evolution
How? You might ask.
Kidneys are the 2 bean-shaped organs that lie in our posterior abdominal wall and are responsible for filtering waste products from our blood to urine. They are in the abdominal region
When studying the kidney during fetal development, you will be surprised by how humans and fish are related. In the fetus, we develop three pairs of kidneys instead of one pair we are familiar with. The first kidney to develop is called pronephros which develops near the neck - yes! you had kidney in your neck when you were fetus, the second one is called mesonephros which develops in the abdomen, and the third pair is called metanephros which develops in the pelvis. The pronephros and mesonephros degenerate and the metanephros is the only one that continues and it gives rise to your kidney. Here how they looked like.
As you see, pronephros and mesonephros degenerate. Metanephros remains.
Here is a zoomed-in picture that shows you how the metanephros gives rise to our kidney.
And here we see that the kidney (which originated from the metanephros) migrates from the pelvis to the abdomen. Remember that the mesonephros is in the pelvis but our kidneys are in the abdominal region. Now it makes sense.
Here comes a critical question. Why did the pronephros and mesonephros form if they would degenerate at the end?
Well, the answer is simple. These things don’t degenerate in fishes. They remain and form important structures. Since we evolved from fishes, we still form these structures. But they regress as we are developing in fetal life. The vertebrate kidney is unique among the organs in that two or three kidney types arise sequentially during fetal stage. In general, fish form the first two kidneys, called the pronephros and mesonephros, with the latter being the permanent adult kidney (mammals develop a third kidney, the metanephros, as their final adult kidney). In fish, the pronephric kidney plays a vital role in osmoregulation, as embryos usually develop externally and are therefore exposed to environmental pressures on water and salt homeostasis.
As we see here, the mesonephros remains in other creatures and it regresses in humans and other creatures and the metanephros is the structure that forms the kidneys.
The next evidence is something called the pharyngeal slits.
Pharyngeal slits are filter-feeding organs found invertebrate chordates. Pharyngeal slits are repeated openings that appear along the pharynx caudal to the mouth. With this position, they allow for the movement of water in the mouth and out the pharyngeal slits. They are formed in humans but regress as we develop!
Footnotes
Some interesting evidence of evolution!
How could our kidneys be a sign of human evolution from sea creatures?