Final answer:
Humans excrete urea, which is more soluble than uric acid. Urea is produced in the liver by the urea cycle, facilitating efficient detoxification and excretion.
Step-by-step explanation:
Humans excrete urea as the primary nitrogenous waste. Urea is formed in the liver through the urea cycle, from the detoxification of ammonia which is generated from the deamination of amino acids. Compared to uric acid, which is excreted by birds, reptiles, and some terrestrial invertebrates, urea is more soluble in water. This high solubility of urea allows for efficient of waste, transportation in bodily fluids, and excretion in the urine. The solubility of these substances is an adaptive trait, with urea being conducive for organisms like humans that have access to ample water, while the less soluble uric acid conserves water for organisms living in drier conditions.