Embryo Development

Birds are descendants of dinosaurs. They carry their DNA. So in its early stages, a chicken embryo will develop dinosaur traits like a long tail, teeth, and three-fingered hands. If you can find the genes that cancel the tail and fuse the fingers to build a wing-and turn those genes off-you can grow animals with dinosaur characteristics.

   -Jack Horner (Wired Magazine: Issue 17.03)

In many cases, the evolutionary history of an organism unfolds during its development. You can clearly watch an embryo exhibiting characteristics of the embryos of its ancestors. That is, one can often observe a structure appearing at one stage during embryonic development that corresponds to an ancestral structure which is no longer present in a species, only to disappear or become modified in a later stage of development. These relict developmental forms suggest strongly that our development has evolved, with new instructions layered on top of old ones. Whales don't have actual legs* or fur but they evolve from land mammals who did. During embryonic development leg extremities first occur early on then later recede. Whales also have hair at one stage during embryonic development but lose most of it later. Dolphin embryos also go through a stage where they grow hind limbs that later recede. Sometimes a feature like the hind limbs of dolphins develop but fail to later recede in the embryo and the animal ends up being born with these ancestral traits. Recently a dolphin that failed to recede its hind limbs before birth was caught off the southwestern coast of Japan. To the right is a photo courtesy of the Taiji Whale Museum. *Whales do still have tiny remnant leg bones that lie buried deep in their bodies. Human Gills The embryos of all humans start to grow gills early in their development. The same is also true with the embryos of all other mammals, birds, reptiles and amphibians since we all share a common ancestry with fish. There is a specific gene in our DNA that tells our embryo to start developing gills, but before the gills are complete another gene later kicks in telling the gills to form into various other structures. The gills on human embryos are not fully developed or used for respiration but neither are the gills of a fish embryo at that age. The gills are still actually legitimate gills, in their early stages, as the slits do establish contact between the outside and the pharynx. The early stages of development for the gills in human embryos mirrors that of fish embryos. It is not until that later gene (which got added to our DNA later in our evolutionary history) kicks in that the gill development takes a different course than that of fish.

All human embryos also go through a stage in which they have tails that are later absorbed. Sometimes however a person is born with an actual fully functional tail. Sometimes the tail is just a long flab of skin that resembles a tail and sometimes the tail has develops muscles making it a fully functional tail.

Do you ever wonder why we have a tail bone? For some people it causes extreme and unnecessary chronic pain known as coccydynia and it hurts terribly if we fall on it. The tail bone, aka the coccyx, dose serve a purpose as an anchor point for the surrounding muscle but wouldn’t it be more intelligent to design an anchor that is more padded and doesn’t cause so much pain if we fall on it or one that doesn’t cause chronic pain? Regardless, the tail bone is a remnant of the embryonic tail that forms in humans of which is degraded and eaten by our immune system before birth. In rare occasions though, some people are actually born with their tail still intact. More than 100 cases of human tails have been reported in medical literature. Most of these are not true functional tails, they are pseudo-tails meaning it is just skin hanging from the tail bone and just looks like a tail. Yet again, where is the intelligence of designing such a birth defect?

In less than one third of the well-documented causes of human tails a person is born with an actual fully functional tail. These true human tails do in fact have a complex arrangement of adipose and connective tissue, central bundles of longitudinally arranged striated muscle in the core, blood vessels, nerve fibers, nerve ganglion cells, and specialized pressure sensing nerve organs (Vater-Pacini corpuscles). They are covered by normal skin with hair follicles, sweat glands and sebaceous glands. These true human tails range in length from about one inch to over 9 inches long and are able to move via voluntary striped muscle contractions in response to various emotional states. Some of these human tails have even contained cartilage and up to five well-developed articulating vertebrae which not even many mammalian tails have.