In utero, we mammals develop three separate kidneys in succession, absorbing the first two before we end up with what will eventually become our adult kidney. The first two kidneys reprise embryonic kidneys of ancestral forms in the proper evolutionary order.
The video below (though in French) shows the production and absorption of the first two ancestral kidneys, as well as the development of the third kidney, the mammalian kidney.
Pronephric kidney (0- 12 seconds)
The Pronephric kidney begins to form at about three weeks in human development. It consists of an organ that in primitive, jawless vertebrates like the hagfish filters wastes from the body cavity and excretes them. The Pronephric kidney does not function in mammals because it begins to disappear shortly after the next kidney forms.
Mesonephric kidney (14 – 40 seconds)
The mesonephric kidney, instead of filtering waste from the body cavity, filters waste from the blood and excretes them through a pair tubes called mesonephric ducts. This kidney eventually will develop into the adult kidney of fish and amphibians. This kidney functions within the human embryo for a few weeks, but also disappears during the final kidney development.
Metanephric kidney (42 seconds – end)
The Metanephric kidney begins development within humans about five weeks into gestation, and consists of an organ like the mesonephric kidney that filters waste from the blood, but excretes them through a pair of new tubes, ureters. In the embryo, the wastes are excreted directly into the amniotic fluid. The Metanephric kidney is the final kidney of reptiles, birds, and mammals.
The development of three kidneys begs for explanation, and it sure does not make a lot of sense through the creationist view. The exclamation through an evolutionary view is the fact that the first two kidneys resemble, in order, those of primitive aquatic vertebrates (hagfish), and aquatic and semi aquatic vertebrates (fish and amphibians) in evolutionary order. We go through developmental stages that show organs resembling those of our ancestors because we are descended from fish and amphibians.