The human yolk sac emerges during the first few days of gestation and provides some of the first cells and circulation necessary for life. After about seven weeks, the yolk sac’s role diminishes as it is supplanted by the placenta.
But in oviparous (egg-laying) animals the yolk sac goes on to be filled with yolk, which allows the fetus to continue feeding while outside the womb.
Because most mammals get their nourishment directly from the placenta and lactation, yolk is unnecessary.
But does the existence of a yolk sac in humans infer evolution?
Evolutionists say yes, insisting the yolk sack is a vestigial remnant of our egg-laying past.
Creationists say no, and point out that the yolk sac plays an essential role in the development of the embryo.
Evolutionists agree that the yolk sac is vital, but argue that it no longer plays the role it once did.
Creationists assert it never played that role, and that God must’ve designed the yolk sac to play two distinct roles.
And historically this is where the debate would end, in a draw, because you can’t prove that human yolk sacs once carried yolk… or can you?
What’s that you’re hiding in your genes?
Recently, researchers decided to see if human DNA contained the genes responsible for creating yolk. I’ll let Professor Dennis Venema explain the particulars:
One protein used as a yolk component in egg-laying vertebrates is the product of the vitellogenin gene. Since placental mammals are proposed to be descended from egg-laying ancestors, researchers recently investigated whether humans retained the remnants of the vitellogenin gene sequence in pseudo-gene form. To assist in their search, this group determined the location of the functional vitellogenin gene in the chicken genome, noted the identity of the genes flanking the vitellogenin sequence, and located these genes in the human genome. They found that these genes were present side-by-side and functional in the human genome; then they performed an examination of the human sequence between them. As expected, the heavily mutated, pseudogenized sequence of the vitellogenin gene was present in the human genome at this precise location. The human genome thus contains the mutated remains of a gene devoted to egg yolk formation in egg-laying vertebrates at the precise location predicted by shared synteny derived from common ancestry.
While the vitellogenin pseudogene is compelling, it is but one example of thousands that could be given.
If creationists are correct in saying that the human yolk sac was never designed to carry yolk, what are we doing with a “scrambled” yolk gene?
The Creationist Response
In light of this new evidence, creationists kick the can further down the road by speculating that the vitellogenin gene is still active, and serves some other purpose in mammals.
It is presumptuous to assume that just because these genes were first found in association with egg yolk that their mammalian counterparts are vestigial remnants or that they, like so much other so-called junk DNA, will not be found to actually have functions unrelated to any ancestral history of making yellow goo.
~ Dr. Elizabeth Mitchell for Answers in Genesis
Dr. Mitchell goes on to point out that the vitellogenin gene is known to serve other purposes in honeybees and some non-mammalian vertebrates.
And maybe she’s right, just because the vitellogenin gene is associated with yolk production in non-mammals, doesn’t mean the gene is associated with yolk production in mammals… well.. except for that it is.
Luckily for us there are a few mammals that actually do have a functional vitellogenin gene; namely the monotremes: the platypus and two species of echidna. And guess what these mammals can do that no other mammal can do? You guessed it, they can produce yolk.
So it doesn’t seem presumptuous to assume that this is what the gene does (or once did) in mammals.
So why has this gene mutated?
It’s worth noting that when a copying error occurs in a vital gene, the carrier becomes less likely to survive or reproduce. But if a copying error occurs in a non-vital gene, the damage is inconsequential, and therefore the error is allowed to remain.
For example, if a chicken were born with a damaged gene that caused its eggs to contain no yolk, this mutation would be an evolutionary dead-end. Nature wouldn’t allow it. But if the same damage occurred in a placental mammal, it would be inconsequential, because the animal could get equivalent sustenance elsewhere.
Natural selection selects for advantages and selects against disadvantages, propelling life forward through a simple process of trial and error. But inconsequential changes are just that.
We have these three clues: an empty yolk sac that’s just like a bird’s, a gene that’s necessary for generating yolk, and evidence that this gene can produce yolk in mammals. Sure, we can always say, “God just wanted to design it this way,” but even if true, God designed it in such a way that also strongly infers evolution.
Professor Dennis Venema writes elsewhere:
These data make perfect sense if humans are descended from egg-laying ancestors and share common ancestry with chickens. It is very difficult to rationalize this data from an antievolutionary perspective. Since the common ancestor of humans and chickens was a reptile, this indicates that the vitellogenin pseudogene should be present in all non-egg-laying mammals. Studies so far have found this unitary pseudogene in wide variety of additional species ranging from dogs to wallabies.
~ Dennis R. Venema, An Evangelical Geneticist’s Critique of Reasons to Believe’s Testable Creation Model: RTB and Human-Ape Common Ancestry
From an evolutionary perspective, our yolk gene has suffered the same fate as our vitamin C gene: both were once vital, but became nonessential after we had alternative sources for obtaining similar sustenance.
Interestingly, creationists argue that our vitamin C gene was once functional in the Garden of Eden, but broke sometime after the fall of man. But creationists can’t very well argue that our yolk gene also once performed perfectly in the garden, so they argue that the gene must do something other than the obvious.
We can always make up new excuses, but if looks like a duck, and quacks like a duck, it probably is a duck. Or in our case, if we have a yolk sac like a duck, and the genes for producing yolk like a duck, then maybe we’re related to the duck.