How the Ape Lost His Tail

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    Many people believe that the theory of evolution contains a claim that humans evolved from monkeys. In fact, the theory includes a claim that humans evolved from apes, not from monkeys. What difference does that distinction make? We see the most obvious difference when we notice that monkeys have tails and that apes donít.

    That observation raises a new question: how did creatures that presumably evolved, as monkeys did, from a lemur-like creature end up with no tail. Like monkeys, lemurs have long tails and so do fossils of earlier primates. Returning to Dreamtime imagery, we ask How Did the Ape Lose His Tail? In confronting that question we have an opportunity to contrast our answer to one of the most famous Just-So stories from the Native American canon.

    Letís start with a relevant anecdote. In July of 2009 I got a new neighbor and noticed that her cat has a tail only a little over half the length of the typical house catís tail. Again, variation shows up to remind us of the fundamental principle of Darwinian evolution Ė descent with modification. So how did that principle change a creature with a long, fluffy tail, like that of a lemur, into a creature with no tail at all?

    Our analysis begins with the fact that Superfamily Hominoidea, the tailless primates (also known as apes), evolved from the other primates 34.5 to 29 million years ago as the Eocene Epoch segued into the Oligocene Epoch, punctuated by a mass extinction. Then 18 million years ago the superfamily split into family Hylobatidae (gibbons) and Hominidae (the great apes [Homo, Pan (chimpanzees), Gorilla, Pongo(orangutan)]).

    The precipitating event was the mass extinction that occurred 33.9 Ī 0.1 million years ago as the Eocene Epoch shaded into the Oligocene. Marked by a large-scale extinction and floral and faunal turnover, it was minor in comparison to the largest mass extinctions. Most of the affected organisms were marine or aquatic in nature and included the last of the ancient cetaceans, the Archaeoceti. In a 1980 paper in Nature John A. OíKeefe pointed to the work of J.A. Wolfe, who used botanical evidence from that period to infer that the climate was marked by normal summers and winters 20 Centigrade degrees colder than normal. Correlating that inference and the contemporary extinction of five species of radiolaria with the North American strewn field (a fall of tectites that sprawls across eastern North America), OíKeefe hypothesized that at the end of the Eocene a thick ring of dust orbited Earth more or less in the equatorial plane.

    A thick enough ring would certainly produce Wolfeís climate pattern. In summer, when Earthís north pole is bowing to the sun, sunlight would reach the Northern Hemisphere unhindered and create normal warming. In winter, when the north pole bows to the stars, the ring would prevent some sunlight from reaching the Northern Hemisphere and would cause that part of Earth to become colder than it would otherwise have become. If the amount of light reaching the ground were diminished by 75%, the deficiency would lead to the required 20-degree extra drop in temperature. It was at this time that Antarctica became covered in ice through a positive feedback effect: as the ice spread over wider areas it reflected more sunlight into space, diminishing the warming of the area and enabling more ice to survive the summer.

    OíKeefe guesstimated that 25 billion tonnes of 100-micron particles would suffice to comprise a ring thick enough to account for the terminal Eocene extinction. Though OíKeefe didnít speculate on the source of the dust, itís fairly obvious whence it came. It helps to remember, when we contemplate cosmic catastrophes, that two rather large targets occupy this part of the solar system. {at 3 tonnes per cubic meter (lunar rock), we need a cube 2 km on a side to provide the 8 billion cubic meters}. An explosion that heaved 25 billion tonnes of dust into space left a large crater that would be quite noticeable if we could see it. It likely lies on the lunar farside, because the youngest large-enough crater on the nearside is Tycho, which has been dated at 108 million years ago.

    The colder winters certainly affected the monkeys in the temperate zone of Southern Africa. One obvious adaptation to the cold makes the animals bigger. A bigger version of a monkey sheds heat less efficiently and, thus, stays warmer. It also generates more heat for a given surface area. As smaller monkeys died in the cold, bigger members of the same species survived and reproduced the mutations that made them bigger.

    But a bigger monkey cannot climb to the tops of the trees: itís too heavy and the branches wonít support it. It must stay lower in the trees and, thus, comes into the hunting range of predators similar to cheetahs. Further, it cannot leap from tree to tree as smaller monkeys do: it must come down to the ground to go from tree to tree. Those facts led to that species of monkey evolving into a new family of tailless primates.

    In mammals the tail functions primarily as a fly swatter. By using their tails to keep flies and other insects off the anal area, the animals reduce the chances of insect bites producing infections. But some mammals (bears and apes primarily) can reach their anal areas with their forelimbs, so they can get by without tails.

    There is a pattern to mammalian tails that offers a clue as to why apes donít have tails. Predators (such as lions and tigers, coyotes and wolves) tend to have long tails while prey animals (such as antelope, sheep, and goats) tend to have short tails. Exceptions to the latter rule include light tree climbers (squirrels and monkeys) and animals that are not easy prey (beavers, porcupines, and skunks for example). Predation gives us a good explanation for that pattern when we notice that a long tail on a prey animal gives a predator a grabhold for bringing that animal down. Predation thus indirectly promotes any mutations that give a prey species shorter tails, doing so by making a long tail a liability.

    Our enlarged Southern African monkey from the Oligocene came under that kind of evolutionary pressure. Thus the monkey came to resemble a gibbon, the most monkey-like of the apes. But it didnít lose the tail entirely: part of the tail still performed a useful function.

    In the apes the tail evolved into the coccyx, which provides a solid attachment for certain muscles, tendons, and ligaments. Consisting of fewer than half a dozen bones, this vestigial tail supports the gluteus maximus on its posterior side (thereby contributing to upright walking and running) and to muscles that move the pelvic floor on its anterior side, thereby contributing to functions such as defecation. Thus we should properly say that apes possess a vestigial tail, rather than say that they lack tails altogether.

    And then that gibbon-like species radiated into the other genera of the ape family. It became true gibbons, orangutans, gorillas, and chimpanzees, the latter of which produced a population that evolved into genus Homo.

    Although we donít have a Just-So story, we have only a plausible explanation of why apes donít have tails. We can accept it only as an hypothesis until paleontologists come up with the evidence to support its elevation to the realm of theory. That evidence would consist primarily of fossils that demonstrate the shortening of primate tails over time and a correlation with other changes in those primatesí bodies, such as larger size and other adaptations for spending more time on the ground. Itís not properly a theory until we have that kind of solid evidence to support it. All we have at this point is what the law calls probable cause, something that can be tested (probed) with evidence. Compare that with something that cannot be proven and either verified or falsified, because thereís no possibility of finding evidence for or against it. Itís one of our favorite Just-So stories from the Native American canon.

How the Bear Lost His Tail:

    Back in the days when the world was young, Bear had a tail and, oh, what a tail it was! It was long and black and glossy and Bear used to wave it around just so that people would look at it and admire him for having it. Bear was just so proud of his tail!

    Fox saw this and certain spirits began to dance inside his heart. Fox, as everyone knows, is a trickster and he loves to play pranks that expose the folly of others. Pride is his favorite target. So he decided to play a trick on Bear.

    It was the time of year when Hatho, the Spirit of Frost, had swept across the land. He blew his cold breath onto the world, turning rain into snow and water into ice. The lake froze over so that the animals could walk on it. It was the perfect place for Fox to play his prank on Bear.

    Fox went out on the lake and cut a hole in the ice, right near a place where he knew Bear liked to walk. Soon Bear came by and he saw, arrayed all around Fox in a big circle, big trout and fat perch. Bear was just about to ask Fox what he was doing when Fox twitched his tail, which he had sticking through that hole in the ice, and pulled out a huge trout.

    "Good day, Brother," said Fox. "Youíre looking well this fine day."

    "Iím feeling well, Brother Fox," answered Bear as he looked at the big circle of fat fish. "I am just wondering what you are doing."

    "I am fishing," said Fox. "And being rather successful at it, as you can see. Would you like to try?"

    "Oh, yes," said Bear, "Yes, I would!" He started to lumber over to Fox's fishing hole.

    But Fox stopped him. "Wait, Brother," he said. "This place will not be a good one for you. As you can see, I have already caught all the fish. Let us make you a new fishing spot where you can catch many big trout."

    Bear agreed that Fox had spoken aright, so he followed Fox to the new place, a place where, as Fox knew very well, the lake was too shallow to catch the winter fish. Those fish always stay in the deepest water when Hatho has covered their ponds and lakes with his shield. Bear watched Fox make the hole in the ice, already tasting the fine fish he would soon catch. "Now," Fox said, "you must do just as I tell you. Clear your mind of all thoughts of fish. If you even think of a song, the fish will hear you. Sit with your back to the hole and place your tail in the water. Soon a fish will come and grab your tail and you can pull him out."

    "But how will I know if a fish has grabbed my tail if my back is turned?" asked Bear. "How do you do it?"

    "It takes practice until you have gained experience," said Fox. "But I will help you until you gain that knowledge. I will hide over here where the fish cannot see me. When a fish grabs your tail, I will shout. Then you must pull as hard as you can to catch your fish. But you must be very patient in order to lure the fish. Do not move at all until I tell you."

    Bear nodded and said, "I will do exactly as you say." He sat down next to the hole, placed his long beautiful black tail in the icy water and turned his back.

    Fox watched for a time to make sure that Bear was doing as he was told and then, very quietly, he took his fish, sneaked back to his own house and went to bed. The next morning he woke up and thought of Bear. "I wonder if he is still there," Fox said to himself. "Likely heís not, but I'll just go and check."

    So Fox went back to the ice-covered lake and at first thought Bear had gone home. Then he saw a little white hill in the middle of the ice. Hatho had come in the night and dusted snow over Bear, who had fallen asleep while waiting for Fox to tell him to pull his tail and catch a fish. And Bear was snoring, snoring so loudly that the ice was trembling. The sight was so funny that Fox rolled on the ground with laughter. Then, when he was through laughing, he decided the time had come to wake up poor Bear. He crept very close to Bear's ear, took a deep breath, and then shouted: "Now, Bear!!!"

    Bear woke up with a start and jerked his long tail as hard as he could. But his tail had been caught in the ice, which had frozen over thickly during the night, so as he pulled his tail, it broke off with a loud snap. Bear turned around to look at the fish he had caught and instead saw his long lovely tail caught in the ice.

    "Ohhh," he moaned, "ohhh, Fox. Ohhh, you sneaky Fox, I will get you for this!" But Fox, even though he was laughing hysterically, was still faster than Bear. With a sudden spring, he leaped aside and was gone.

    So even to this day bears have short tails and no love at all for foxes. And if you ever hear a bear moaning, it is likely because he remembers the trick Fox played on his long-ago ancestor and he is mourning for his lost tail.

    Of course, evolution would tell a different story, but thatís a topic for another day.

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