Question

Humans are bipedal, terrestrial omnivores. As a species, we are adapted to quite a range of habitats, from deserts to mountains and from rain forests to tundra. Ultimately, however, we are descended from early hominids who evolved in African plains, and our body systems tend to reflect this. Your job is to determine how humans might be adapted if they had evolved in an aquatic environment where they live entirely in the water and breathe through gills.

Consider the following:

• What would be required to extract oxygen and exchange CO₂ and how the circulatory would have to change to compensate? You are encouraged to look up other (non-human; can be non-mammalian) animals that share these habitats for ideas.
• What if humans had evolved and were adapted to live in very specific habitats? In other words, it would still look *basically* like a human but would have some important differences related to its circulatory and/or breathing systems.
• What would these systems of a human that adapted to a different environment look like, and why would it look like this?
• You should consider the following parts of the breathing and/or circulatory system in your answer: Heart – size, activity, Lungs - size, Breathing rate, Oxygen transport system, Blood - Red Blood Cells (count), hemoglobin, and plasma levels, anything else you think is important to the circulatory and breathing systems.

Answer 1

First of all we need to understand about the evolution of animals, vertebrate evolved from invertebrate and human belongs to class mammals which is on the top of evolution tree and most advanced animal. Since living is aquatic environment is completely different even some mammals predominantly live in the aquatic enviroment like Whales and Dolphins but they do have lungs for the respiration and take oxygen from air not from water. In case of exclusively aquatic animals like fishes where they are adapted for aquatic respiration and extract dissolved O2 from water, which is really very as compared to air (less than 1%). Fishes are having gills (4 to 5 pairs in bony or cartilagibous fishes) and this organ is specialized for extraction of dissolved O2 from water and gaseous exchange takes place in gill lamellae though counter current exhange (O2 and CO2 exhange). This counter current exchahge (blood flow in the opposite direction of water flow) is so effecient and provide maximum O2 extraction through diffusion as compared to co current flow. This phenomenon is very effecient in venous blood flow where only two chamber in the heart present. Another adaptation is streamline body in aquatic organism for bouyancy in the aquatic environment and also osmoregulation in aquatic environment also (even it also varies in different water bodies like fresh, brackish or marine water where they have challange to maintain the salt concentration too). Cellular and non-cellular component of blood is equally important (RBCs, WBCs and plasma) and also need modification in kidneys for proper excretion. Conclusively, living in aquatic environment would be more challanging as on lands and in evolution, amphibians evolved from fishes, reptites from amphibians, aves (birds; warm blooded) evolved from reptiles (cold blooded) and mammals evolved from aves (birds). We may see similarties as well as difference in each class.