Consequently, in humans the urethra and vagina have separate external openings. Urethra, ovaries, uterine tubes, labia, mesenteries, testes, epididymis, vas deferens, inguinal canal, prostate gland, etc. Bicarotid trunk — In fetal pigs, the brachiocephalic artery splits into the right subclavian artery and the bicarotid trunk. The bicarotid trunk then splits into the right and left common carotid arteries. Humans do not have a bicarotid trunk; instead, the left common carotid artery branches from directly from the aorta, while only the right common carotid artery originates from the brachiocephalic artery.
Illiac arteries -Humans have a common illiac artery which branches into internal and external illiac arteries. Fetal pigs do not have a common illiac artery. Instead, the internal and external illiac arteries branch directly off of the aorta. Sheep heart — The sheep heart is very similar in size and shape to the human heart — slightly larger than the average human heart, but otherwise very similar.
Sheep eye — The sheep eye is identical to the human eye in all major respects. Sheep brain — The sheep brain has the same basic plan as all mammalian brains, including humans.
Compared to a human brain, the sheep brain has a relatively smaller cerebral cortex and consequently a relatively smaller overall size. The other major parts are identical in terms of gross anatomy. If you look at the back wall of a fetal pig heart, at the location where the coronary sinus is found in a human heart or a sheep heart, you will see a vessel that is enormous compared to the relatively small size of the fetal pig heart.
In humans, there is an azygous vein which develops a connection to the heart through the right common cardinal vein which becomes the superior vena cava in the adult. In humans, the proximal left common cardinal vein becomes very small, draining only the heart wall, as the coronary sinus.
The hemiazygous vein develops a connection to the azygous vein through an anastamosis. In pig development, there is no azygous vein, and the hemiazygous vein drains both sides of the posterior chest wall. The hemiazygous vein loses its connection to the right common cardinal vein, and maintains its connection through the left cardinal vein. The left cardinal vein, consequently, drains not only the heart wall, but also the posterior chest wall through the hemiazygous vein. Consequently, the left cardinal vein becomes very large in pigs.
So do these similarities mean that humans are closely related to pigs? Not necessarily. Many of these shared physical traits are not the result of a close ancestry, but rather of convergent evolution—that is, selection of the same characteristics by a common environment.
But what about on the genetic level? Well, that might be a different story. Comparison of the full DNA sequences of different mammals shows that we are more closely related to mice than we are to pigs.
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