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In a letter to the American botanist Asa Gray, written a year after the publication of The Origin of Species, Charles Darwin urged that "embryology is to me by far [the] strongest single class of facts in favour"1 of his theory of evolution. A lot has changed in the 150-plus years since Darwin penned those words, but embryology remains a favorite line of evidence used by evolutionists to bolster common descent.
Open almost any high-school or college-level general biology textbook from the past few decades, and turn to the section on evolution. There you will find some diagram—perhaps using drawings, perhaps showing photographs—portraying the embryos of different species of vertebrates as highly similar to each other. These early similarities in vertebrate development are said to reflect the species' common ancestry.
Here are two examples of the kind of language typically used in these textbooks:
Chordate embryos . . . appear very similar in the first stages of development . . . evidence that they share a common ancestor that developed along the same pathway.2
Early in development, the human embryos and the embryos of all other vertebrates are similar. These early similarities are evidence that all vertebrates share a common ancestor.3
These claims are wrong, but before discussing why, we must revisit some of the darker history of embryological thought.
If you took high-school biology in, say, the mid-twentieth century, you might expect the textbooks quoted above to also say something like, "ontogeny recapitulates phylogeny." This idea, called recapitulation theory, was popularized by the prominent German biologist Ernst Haeckel, who sought to bolster Darwin's ideas in the years after the publication of The Origin of Species.
Haeckel's theory taught that the development of an organism ("ontogeny") replays ("recapitulates") its evolutionary history ("phylogeny"). Since the standard evolutionary view holds that humans evolved from fish, recapitulation theory taught that, at one point between your conception and birth, you went through a "fish stage."
Biologists now know that vertebrate embryos do not replay their supposed earlier evolutionary stages, and firmly hold that the recapitulation theory is false.4 The concept has been completely removed from current textbooks, but it was taught to biology students as fact for decades, and many people still believe it's true.
Anyone can inadvertently pass along a bad idea, right? Well, there's a darker side to recapitulation theory. Not only was the concept wrong, but its means of promotion—through embryo drawings concocted by Haeckel (see beginning of article)—were fraudulent. According to the journal Science, "generations of biology students may have been misled" by Haeckel's phony drawings, which were commonly reproduced in biology textbooks.5
The main problem with these drawings is that they overstate the degree of similarity between embryos of different vertebrates in their earliest stages. As Stephen Jay Gould explained, Haeckel's methods "can only be called fraudulent" because he "simply copied the same figure over and over again"6 when depicting the early embryos of different species. Leading embryologist Michael Richardson called the drawings "one of the most famous fakes in biology."7
Not to worry, the New York Times reassured us in 2008. While reprinting material from the National Center for Science Education, the newspaper admitted that Haeckel's drawings were "long-discredited," but claimed they hadn't been used in textbooks since "20 years ago."8
If only that were true. Multiple recent biology textbooks have used Haeckel's drawings, including one published by the Darwin-defending geologist Donald Prothero in 2013.9 Such reuse of Haeckel's drawings led Gould to exclaim:
[W]e do, I think, have the right to be both astonished and ashamed by the century of mindless recycling that has led to the persistence of these drawings in a large number, if not a majority, of modern textbooks!10
But there is some good news here. In 2000, biologist Jonathan Wells published Icons of Evolution: Science or Myth? Why Much of What We Teach About Evolution Is Wrong (Regnery), which raised public awareness about Haeckel's fraud, ultimately forcing many (though apparently not all) publishers to remove Haeckel's drawings from their textbooks. That doesn't mean the problem is fixed, however. Most textbooks still use other figures to portray the early stages of vertebrate development as highly similar—leading to the sorts of textbook captions cited at the beginning of this article.
But are those captions really inaccurate?
Yes, they are. Biologists who investigate embryology have found considerable differences among vertebrate embryos from their earliest stages onward, contradicting what we are told to expect from common ancestry.
Two of the earliest stages of vertebrate development are cleavage and gastrulation. During cleavage, a newly fertilized zygote undergoes rapid cell replication until the embryo becomes a tiny ball of cells, with the basic axes that will define the body plan laid out. Next, during gastrulation, the embryo begins to increase in size while forming distinct germ layers, which will later develop into individual organs. A 2000 paper in Systematic Biology states that even such "early stages as initial cleavages and gastrula[tion] can vary quite extensively across vertebrates."11
Likewise, a 2010 paper in Nature states: "Counter to the expectations of early embryonic conservation [i.e., similarity], many studies have shown that there is often remarkable divergence between related species both early and late in development."12 Or, as a 1997 article in Trends in Ecology and Evolution stated, "despite repeated assertions of the uniformity of early embryos within members of a phylum, development . . . [in those early stages] is very varied."13
Rather than looking highly similar in their early stages, actual vertebrate embryos look more as depicted here.
When Darwin cited embryology as the "strongest single class of facts" in favor of his theory, he didn't know what we know today about embryology. What excuse do biology textbook publishers have for continuing to overstate the case for common ancestry through embryology? •
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