This is a blog about paleontology (the study of the history of life on Earth through the fossil record) with an emphasis on vertebrate paleontology, the study of extinct vertebrates (animals with backbones). The methodology and findings of paleontology will be discussed, as well as related issues such as evolutionary theory. The blogger is a vertebrate paleontologist specializing in the Triassic Period, the Beginning of the Age of Dinosaurs.
My posts are presented as opinion and commentary and do not represent the views of LabSpaces Productions, LLC, my employer, or my educational institution.
Last time, we introduced the nested classification of Linnaean taxonomy originally created in the mid-18th century by Christian creationist Carl Linnaeus. Using this classification system, we established that human beings are not only animals, but vertebrates…even if we completely avoid saying the word “evolution” and just look at our overall anatomy. Vertebrates, you make recall, are eukaryotes (they have cells with a nucleus), animals (mobile multicellular eukaryotes which eat the cells of other organisms), eumetazoans (animals with organized tissues), bilaterians (eumetazoans with bilateral symmetry), deuterostomes (bilaterians in which the opening for the anus develops before that for the mouth), chordates (deuterostomes which have a notochord, pharyngeal pouches, and a tail), craniates, and vertebrates (chordates with an internal skeleton protecting the brain and spinal chord).
Vertebrates include things that we call "fish," as well as amphibians (like frogs and salamanders), "reptiles" (like lizards, snakes, and crocodiles), mammals, and birds. However, several types of "fish" lack a couple of features possessed by most: notably jaws and a skull and backbone reinforced by true bone (which is hardened by a mineral called calcium hydroxylapatite). Under Linnaean taxonomy, these jawless fish (which include modern lampreys and hagfish) are put in a superclass called Agnatha. Other vertebrates (including most fish), which all have jaws, are put in superclass (or infraphylum) Gnathostomata. Humans have jaws, in case you hadn’t noticed.
Some jawed vertebrates, like agnathans, have internal internal skeletons made almost entirely or cartilage, and only have calcium hydroxylapatite reinforcing the teeth and part of the vertebrae. These are placed in a class or superclass called Chondricthyes (the sharks, rays, and chimeras). Other gnathostomes have the internal skeleton (including the skull and backbone) extensively reinforced with calcium hydroxylapatite, which also includes a hard ribcage to protect the internal organs; these "bony fish" and all other vertebrates are placed in a superclass called Osteicthyes. Humans have hard bones of calcium hydroxylapatite, including a rib cage.
Among jawed vertebrates with bony skeletons, one big group called Tetrapoda ("amphibians", "reptiles", mammals, and birds) share more similarities with certain kinds of fish than with others. Some bony "fish" have fins in which the bones are more massive than in most fish, and have a branching pattern in which more and more bones branch off the further you get from the body; you see this in the limbs of tetrapods too. For example, you have one upper arm bone (the humerus), two bones in your forearm (the radius and ulna), and a radiating series of five digits in your hand. You have the same pattern in your legs and feet. Tetrapods and fish with this branching pattern (so-called “lobe-finned” fish) are grouped together in a group called Sarcopterygii.
However, in tetrapods these bones are much sturdier than in any fish, and for a good reason: tetrapods use their "fins" to support themselves on dry land. Moreover, tetrapods have a shoulder blade which does not contact the skull, whereas in bony fish, it usually does. This is because most tetrapods are quadrupeds (they walk using their forelimbs), and if the shoulder was attached to the head, the shock of taking steps would be transmitted to the skull. Moreover, in most tetrapods the hips are firmly attached to the backbone to help support the body on dry land; in fish, the hips are separated from the backbone. Your shoulder blade is separate from your skull, and your hip is attached to you backbone. You can see these features, as well as the branching limb bones characteristic of sarcopterygians, in the human skeleton below.
Moving on! Among tetrapods, the group we sometimes call "amphibians" (frogs and salamanders) have soft gooey skin and lay soft gooey eggs, which both need to be kept moist to survive; amphibians are tied to life in the water. They also have to reproduce in the water, because the male doesn't fertilize the eggs while they are inside the female's body. The female lays the eggs first and the male releases sperm over them, so they have to swim to the eggs outside the female; this therefore has to happen in wet conditions, and is called external fertilization. Most fish do it the same way. Moreover, the eggs in both fish and amphibians generally don't contain very much food (in the form of yolk) for the developing embryo, so they usually do not develop very long in the egg before they have to hatch and go looking for food on their own.
In contrast, all other tetrapods fall into a group called Amniota, which use internal fertilization (the fun kind) and most lay a very special kind of egg, the amniotic egg. This egg contains a membrane called the amnion, which protects the embryo, particularly by keeping it from drying out. Amniotic eggs also usually have a much bigger yolk supply to feed the embryo, so that they can develop longer before hatching. However, an amnion and yolk are found in tetrapods that develop the embryo inside the mother's body instead of in an egg...including humans. During development, the human embryo is surrounded by an amnion and has a small yolk sac connected to the embryo in the exact same area as in egg-laying tetrapods, although it serves a slightly different function, being where the circulatory system begins to develop. See the black and white drawing of a human embryo below.
There are three surviving groups of amniotes which are traditionally recognized in Linnaean taxonomy: Reptilia, Aves (birds) and Mammalia (there are actually slightly bigger and more inclusive groups within Amniota that these three surviving groups fall into that we will talk more about them later). You probably know most of the basic differences between these groups. Most mammals (unlike reptiles and birds) do not lay amniotic eggs but develop the embryo inside the mother's body, lack scales and/or feathers, have hair, have mammary glands usually concentrated into nipples for producing milk (which female mammals use to feed their babies). They also have three bones in the middle ear which are used to transmit sound from the eardrum to the inner ear (reptiles and birds only have one), and have heterodont dentition, meaning that they have very differently shaped teeth in their mouth for different kinds of food processing (i.e. incisors, canines, premolars, molars). Reptiles generally have homodont dentition, meaning their teeth tend to be a lot more similar to each other, and birds of course don't have teeth at all (at least, the living ones don't...more on that later). Humans, of course, have hair, heterodont dentition, and three middle ear bones (we call them the hammer, anvil, and stirrup, or the malleus, incus, and stapes). The image to the left shows the different kinds of teeth in the human mouth.
One group of mammals, the monotremes (the duckbilled platypus and echidna), lay amniotic eggs in which the young are fed by a yolk sac. Another group, the marsupials (e.g. opossums, kangaroos), do not lay eggs. Instead, the embryo is initially gestated inside the mother's womb. The embryo is partially fed by a yolk sac; however, there is also an organ called a placenta which connects the mother's bloodstream to the embryo, so that nutrients in the mother's bloodstream can be passed to the embryo. However, the marsupial placenta can't keep the mother's immune system from eventually attacking the embryo through the mother’s bloodstream (the embryo is, after all, half made from the father's DNA, and therefore seen by the mother's immune system as an invading organism). Therefore, the young disconnect from the placenta and are born after only a few weeks after conception while still an underdeveloped embryo, crawl up to a special pouch on the mother's belly where the nipples are located, latch onto one, and finish development in the pouch getting nutrients from milk instead of the mother's bloodstream.
The biggest group of mammals are the placentals, in which the placenta is a much more elaborate structure that discourages the mother's immune system from attacking the embryo (though not always effectively; human miscarriages are sometimes caused by an immune system response, although this is not the most common cause). The placenta therefore allows the embryo to develop longer in the womb. The embryo may stay inside the mother for several months, or even (in the case of elephants) a couple years. Humans develop with the help of a placenta for about nine month; they do not lay eggs or have a pouch like a kangaroo.
Now we are really getting down to it. In spite of the fact that they share the above characteristics, mammals are an incredibly diverse group; think about whales, horses, elephants, bats, monkeys, cats, moles, monkeys...mammals occupy an incredible range of body plans, habitats, and lifestyles. One group is called Primates, and includes prosimians (lemurs and their relatives), monkeys, and apes. Primates have extremely large brains relative to the size of their bodies, and are generally more intelligent than other mammals. Most primates live in trees, and have a variety of features useful for life climbing around in the branches. They have forward facing eyes, which gives them overlapping fields of vision (both eyes see the same thing); in most mammals (a horse for example), the eyes face sideways, which means that the left and right eyes just see what is on the left and right side of the animal, which little or no overlap. Sideways facing eyes are useful for seeing as wide a field of view as possible (a horse can see you coming up from behind with its peripheral vision) which is good if something what wants to eat you might be sneaking up on you. Most primates, living in trees, don't have to worry about this quite as much...a much bigger problem for them is falling. Having overlapping fields of view gives stereoscopic, or 3-D vision, allowing them to judge distances, which is a critical skill if you are jumping and swinging around in the branches. Forward facing eyes giving 3-D vision are shared with a another group of mammals called Carnivora (e.g. dogs and cats), who use it to judge the distance to their prey. However, most primates also have color vision, something that dogs and cats lack. However, primates have a relatively poor sense of smell compared to most other mammals. Most primates also have opposable thumbs (a thumb which can touch its tip to the other fingers, allowing us to grab things), and have flat fingernails instead of claws. You have forward-facing eyes with color vision, a pretty poor sense of smell compared to most mammals, an opposable thumb, and fingernails. Notice that you can see these features in the macaque below.
Within primates, the group we call apes (the taxon name is Hominoidea) has several features setting them apart from other primates. They tend to be much larger than prosimians and monkeys, and have even bigger brains. They also have a much more flexible arm and shoulder; the shoulder blade in most primates (and most other vertebrates) is on the side of the ribcage, while in apes, it is on the back (see the drawing of the back view of the human skeleton above; the scapula is the shoulder blade). This movement of the shoulder blade allows apes to stretch their arms straight up above their heads, something that other primates can't do, and apes use this to swing from branches instead of walking along them, the same way we swing from "monkey" bars. Like all chordates, apes have a tail as embryos, but differ from nearly all others in that they lose it before they are born. Yep, that’s you.
Why do we fall so tidily into this classification system? Why couldn't we be a hodge-podge of anatomical traits from all over the animal, plant, and fungus kingdoms? Is there any good reason why we couldn't have a cell without a nucleus, photosynthesize like a plant, develop from a blastula in which the blastopore forms the mouth, have a hard external skeleton like a crayfish, have a tail like a monkey? We would totally buck the relatively nice and tidy Linnaean hierarchy that all life falls into...but we don't. We have multicellular heterotrophic eukaryotes with distinct tissues and bilateral symmetry, an embryo in which the blastopore develops into the anus, and at one point possesses a notochord, postanal tail, and pharyngeal slits, an internal skeleton with a skull, backbone, jaws, and ribs all reinforced with calcium hydroxylapatite, sturdy branching limb bones in which the shoulder girdle is separate from the skull and the pelvis is attached to the backbone, our embryos develop for months in the womb with the help of a placenta, we have hair, nipples, three inner ear bones, heterodont dentition, forward facing eyes with stereoscopic color vision, opposable thumbs and fingernails, have a flexible shoulder girdle, loose our tail before birth. We are eukaryotes, opisthokonts, animals, eumetazoans, bilaterians, deuterostomes, chordates, craniates, vertebrates, gnathostomes, osteichthyans, sarcoptergians, tetrapods, amniotes, mammals, placentals, primates....and apes. Our anatomy places us squarely in every one of these groups, with no ambiguity. In the interest of brevity, I didn't list every single trait that puts us in these groups, and I even skipped over a bunch of subcategories (for example, Catarhinni, the group of primates containing apes and Old World Monkeys,and excluding New World Monkeys) that we fall into just as nicely.
We don't need a single piece of evidence from the fossil record to tell us that we are descended from apes (although we have a truckload), because we ARE apes, and so were our parents and grandparents. And you may have noticed that I never even said a peep about evolution anywhere in this post. The patterns jump out, regardless of the explanation for how they came about. The only really unusual thing about us as animals is our behavior and intellectual capacity, and that changes nothing about how our anatomy falls into the Linnaean hierarchy. We may (or may not) be the most intelligent, spiritual, elevated creatures on the planet...but we still have the bodies of animals. And not just any animals, but one very particular group of animals. It is ridiculous to claim, as creationists often do, that this dehumanizes us. Saying that human beings are apes, primates, and animals does not make us less human...any more than saying that a parrot is a bird makes it less of a parrot. Part of being a human being means being an ape.
Even if evolution is completely wrong and we were intelligently designed…we were intelligently designed as apes. Even a Christian creationist could see that. Carl Linnaeus was not thrilled about his own conclusions on how to classify humans, but he couldn't ignore the evidence. In response to critics, he wrote:
It does not please [you] that I've placed Man among the Anthropomorpha [what he would later called Primates] but man learns to know himself. Let's not quibble over words. It will be the same to me whatever name we apply. But I seek from you and from the whole world a generic difference between man and simian that [follows] from the principles of Natural History. I absolutely know of none. If only someone might tell me a single one!
It's a shame that modern creationists don't show the same level of courage and honesty.
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Another spectacular post. I love all of the pokes at creationism.
I am essentially a creationist but I agree that many creationists are extremely intellectually dishonest when recognising the simple facts. Yes, we have the same "floor-plan" as apes, in the same way that the Renault Fuego had the same floorplan as the Renault 14 (oh, never mind!). I mean, there would be no reason for a Creator not to reuse effective designs, even for his favourite creation!