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Authors: Gary Parker
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these so-called "gill slits" (pharyngeal pouches) are quite essential for distinctively human development.
As with "yolk sac," "gill slit" formation represents an ingenious and adaptable solution to a difficult engineering problem. How can a small, round egg cell be turned into an animal or human being with a digestive tube and various organs inside a body cavity? The answer is to have the little ball (or flat sheet in some organisms) "swallow itself," forming a tube which then "buds off" other tubes and pouches. The anterior pituitary, lungs, urinary bladder, and parts of the liver and pancreas develop in this way. In fish, gills develop from such processes, and in human beings, the ear canals, parathyroid, and thymus glands develop. Following DNA instructions in their respective egg cells, fish and human beings each use a similar process to develop their distinctive features (see Figure 8).
Figure 8. Far from being "useless evolutionary leftovers," the mis-named structures above are absolutely essential for normal human development. Similar structures are used for different functions in other embryos — and we normally consider variation on a theme and multiple uses for a part as evidence of good creative design.
What about the "tail"? Some of you have heard that man has a "tail bone" (also called the sacrum and coccyx), and that the only reason we have it is to remind us that our ancestors had tails. You can test this idea yourself, although I don't recommend it. If you think the "tail bone" is useless, fall down the stairs and land on it. (Some of you may have actually done that — unintentionally, I'm sure!) What happens? You can't stand up; you can't sit down; you can't lie down; you can't roll over. You can hardly move without pain. In one sense, the sacrum and coccyx are among the most important bones in the whole body. They form a crucial point of muscle attachment required for our distinctive upright posture (and also for defecation, but I'll say no more about that).
So again, far from being a useless evolutionary leftover, the "tail bone" is quite important in human development. True, the end of the spine sticks out noticeably in a one-month embryo, but that's because muscles and limbs don't develop until stimulated by the spine (Figure 8). As the legs develop, they surround and envelop the "tail bone," and it ends up inside the body.
Once in a great while there are reports of a child born with a "tail." Since the parents were quite pleased, one such child born recently in India was featured prominently on TV news in 2005. But was it really a tail? No, it's just a bit of skin and fat that tells us, not about evolution, but about how our nervous systems develop. The nervous system starts stretched out open on the back. During development, it rises up in ridges and rolls shut. It starts to "zipper" shut in the middle first, then it zippers toward either end. Sometimes it doesn't go far enough down, and that produces a serious defect called
spina bifida
. Sometimes it rolls a little too far. Then the baby will be born,
not
with a tail, but with a fatty tumor. It's just skin and a little fatty tissue, so the doctor can just cut it off. It's not at all like the tail of a cat, dog, or monkey that has muscle, bones, and nerve, so cutting it off is not complicated. (So far as I know, no one claims that proves we evolved from an animal with a fatty tumor at the end of its spine.)
Unfortunately, evolution has such a hold on our thinking that doctors hate to tell a mother if she has a baby with a "tail." They can imagine the dismay: "Oh no; I've given birth to a throwback to the monkey stage in evolution!" Then the arguments begin: "It's your side of the family." "No, it's your side!" Fortunately, the extra skin and fat is not a tail at all. The details of human development are truly amazing. We really ought to stop, take a good look at each other, and congratulate each other that we turned out as well as we did!
There
As with "yolk sac," "gill slit" formation represents an ingenious and adaptable solution to a difficult engineering problem. How can a small, round egg cell be turned into an animal or human being with a digestive tube and various organs inside a body cavity? The answer is to have the little ball (or flat sheet in some organisms) "swallow itself," forming a tube which then "buds off" other tubes and pouches. The anterior pituitary, lungs, urinary bladder, and parts of the liver and pancreas develop in this way. In fish, gills develop from such processes, and in human beings, the ear canals, parathyroid, and thymus glands develop. Following DNA instructions in their respective egg cells, fish and human beings each use a similar process to develop their distinctive features (see Figure 8).
Figure 8. Far from being "useless evolutionary leftovers," the mis-named structures above are absolutely essential for normal human development. Similar structures are used for different functions in other embryos — and we normally consider variation on a theme and multiple uses for a part as evidence of good creative design.
What about the "tail"? Some of you have heard that man has a "tail bone" (also called the sacrum and coccyx), and that the only reason we have it is to remind us that our ancestors had tails. You can test this idea yourself, although I don't recommend it. If you think the "tail bone" is useless, fall down the stairs and land on it. (Some of you may have actually done that — unintentionally, I'm sure!) What happens? You can't stand up; you can't sit down; you can't lie down; you can't roll over. You can hardly move without pain. In one sense, the sacrum and coccyx are among the most important bones in the whole body. They form a crucial point of muscle attachment required for our distinctive upright posture (and also for defecation, but I'll say no more about that).
So again, far from being a useless evolutionary leftover, the "tail bone" is quite important in human development. True, the end of the spine sticks out noticeably in a one-month embryo, but that's because muscles and limbs don't develop until stimulated by the spine (Figure 8). As the legs develop, they surround and envelop the "tail bone," and it ends up inside the body.
Once in a great while there are reports of a child born with a "tail." Since the parents were quite pleased, one such child born recently in India was featured prominently on TV news in 2005. But was it really a tail? No, it's just a bit of skin and fat that tells us, not about evolution, but about how our nervous systems develop. The nervous system starts stretched out open on the back. During development, it rises up in ridges and rolls shut. It starts to "zipper" shut in the middle first, then it zippers toward either end. Sometimes it doesn't go far enough down, and that produces a serious defect called
spina bifida
. Sometimes it rolls a little too far. Then the baby will be born,
not
with a tail, but with a fatty tumor. It's just skin and a little fatty tissue, so the doctor can just cut it off. It's not at all like the tail of a cat, dog, or monkey that has muscle, bones, and nerve, so cutting it off is not complicated. (So far as I know, no one claims that proves we evolved from an animal with a fatty tumor at the end of its spine.)
Unfortunately, evolution has such a hold on our thinking that doctors hate to tell a mother if she has a baby with a "tail." They can imagine the dismay: "Oh no; I've given birth to a throwback to the monkey stage in evolution!" Then the arguments begin: "It's your side of the family." "No, it's your side!" Fortunately, the extra skin and fat is not a tail at all. The details of human development are truly amazing. We really ought to stop, take a good look at each other, and congratulate each other that we turned out as well as we did!
There
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