Mammals evolved during the Triassic period around 250 MYA. At this time the earths’ oxygen levels were about 50% less than that of today and even lower than that during the Jurassic period which gave rise to the emergence of birds. Under these environmental conditions, natural selection favored the loss of the nuclei in mammalian red blood cells. The absence of a nucleus allows for a greater volume of hemoglobin to be present and increases the rate of oxygen saturation. This also makes the red blood cells more flexible
and able to pass through narrow capillary beds.
Mammalian lungs consist of alveoli, tiny air sacs that allow for gas exchange. Gas exchange in birds occurs thru flow tubes called air capillaries which increases the efficiency of respiration delivering more oxygen per breathe. Therefore was no selective pressure to eliminate the nuclei from birds’ red blood cells. This is however not without a cost as toxins are also more rapidly transferred. Coal miners used to take caged canaries down into the mines. If toxic gases such as carbon monoxide were present the canary would die giving the miners time to escape. The blood cells of other types of organisms’ reptiles and amphibians are also nucleated.
Using the key determine which of the slides is human, rabbit (mammalian), frog, bird, or fish.
Select a mammalian blood slide
Sketch a red blood cell and a white blood cell (label the nucleus). Record the total magnification.