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The Origin of Human Chromosome 2: Another Look

By Guest Writer - December 6, 2018
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By Hugh Henry and Daniel J. Dyke

In a September 2018 article, Fazale Rana discredited what is purported to be evidence for an evolutionary theory that humans descended from an “ancestral ape.”

The evolutionary argument posits that “two ancestral ape chromosomes [chimpanzee chromosomes 2A and 2B] fused to give rise to human chromosome 2” in what is referred to as an “ancient telomere-telomere fusion.”1 Proponents cite two recent articles in Nature as evidence for this hypothesis. The articles report how two teams of synthetic biologists successfully performed multiple telomere-telomere fusions and produced functional organisms in brewer’s yeast.2

Rana argues these articles actually undermine the chimpanzee-to-human hypothesis. Because the synthetic biologists had to undertake such extensive and precise gene editing, Rana contends that there is no way such fusions could have occurred via undirected random processes. Nevertheless, some might challenge Rana’s argument by claiming “anything can happen, given enough time.” We propose to address such claims through a series of “what if” scenarios.

What If Spontaneous Fusion Happened?

Let’s suppose chimpanzee chromosomes 2A and 2B truly did fuse spontaneously into human chromosome 2 in the distant past? Would the fusion have resulted in viable humans that outcompeted “ancestral apes” to become a dominant species?

First, it should be understood that telomeres are a DNA sequence that terminate chromosomes to provide protection and stabilization. Chromosomes occasionally break, and a broken chromosome can readily combine with another broken chromosome. One function of telomeres is to prevent intact chromosomes from fusing with either broken chromosome fragments or other intact chromosomes.

Yet the scientific literature reports that telomeres occasionally fail in this function, and fusion of intact chromosomes can occur. A protein complex known as shelterin appears to be critical to the protection process because when shelterin is absent telomeres can undergo end-to-end fusion. The result is genomic instability and possibly carcinogenesis.3 Furthermore, a 2013 article in Nucleic Acids Research reports, “Functional telomeres can be involved in spontaneous telomere fusions [in yeast, . . . resulting in] chromosome instability and may underlie early tumourigenesis” (emphasis added).4 The authors also reported that such spontaneous fusion events are rare.

This research suggests that if ancient chimpanzee chromosomes 2A and 2B did indeed undergo a telomere-telomere fusion into human chromosome 2, the likely outcome would have been genetic instability which might possibly lead to cancer. We could argue, then, that it is extremely unlikely that viable humans would have been the result.

Yet this argument might also not be sufficient for some. So suppose such a spontaneous telomere-telomere fusion did occur and produced a viable human chromosome 2. What would be the outcome?

What If Fusion Produced Viable Humans?

Humans and chimpanzees have a common reproductive process. Offspring receive two, paired versions of each chromosome: one from the father and one from the mother. During reproduction (meiosis), the pairs split into sperm (male) or ovum (female) “gametes” having half the number of chromosomes. Sperm and ovum gametes then come together, with each gamete contributing half the chromosomes to form a new individual. However, whereas human gametes each have 23 chromosomes, chimpanzee gametes have 24.

Suppose an ancient chimpanzee experienced a telomere-telomere fusion of chromosomes 2A and 2B into a viable human chromosome 2. In the reproductive process, it would generate a sperm or ovum gamete with 23 chromosomes—but what would it “mate” with? What is the likelihood that the same thing happened spontaneously with another chimpanzee of the opposite sex and that these two creatures with human chromosome 2 just happened to reproduce with each other? That seems intuitively improbable! Thus it would be more likely that the hypothetical 23-chromosome gamete would have had to join with a standard 24-chromosome chimpanzee gamete. There are historical experiments and other scenarios that illustrate the probable outcome of such a mating.

In the 1920s, Russian zoologist Ilia Ivanov, an expert in artificial insemination, sought to create an ape-human hybrid with the financial backing of the Russian Bolshevik government and the American Association for the Advancement of Atheism.5 He succeeded in inseminating three female chimpanzees with human sperm, but no successful fertilization occurred.6 His efforts went no further, as he became the victim of a Stalinist purge.

Ivanov’s three unsuccessful efforts do not prove that it is impossible to unite a 23-chromosome gamete produced by telomere-telomere fusion with a normal 24-chromosome chimpanzee gamete. Nevertheless, the notion resembles the well-known case of mating a male donkey (31 pairs of chromosomes) with a female horse (32 pairs) to produce a mule. Mules cannot reproduce—or at least they reproduce so infrequently that it is big news when it does occur. In humans, an extra copy of chromosome 21 results in Down syndrome. Hence, it is unlikely that such an ancient chimpanzee union could have produced a viable human.

This unlikelihood is borne out by the synthetic biologists who reduced yeast chromosomes. They found a “reduction in gamete production and viability in meiosis”7 in the yeast cells with reduced numbers of chromosomes. They also found that these modified yeast cells were unlikely to reproduce with “wild,” unmodified yeast.

Rana concludes that “the fusion of yeast chromosomes in the lab makes it hard to think that unguided evolutionary processes could ever successfully fuse two chromosomes, including human chromosome 2, end on end.” To this we add that even if such a fusion event did occur in the distant past, it is hard to think that the result would have been viable, reproducing humans. Creation appears to make even more sense.

Please check out the biography of RTB’s Visitng Scholars: Hugh Henry and Dan Dyke.

Endnotes
  1. J. W. IJdo et al., “Origin of Human Chromosome 2: An Ancestral Telomere-Telomere Fusion,” Proceedings of the National Academy of Sciences 88, no. 20 (October 15, 1991): 9051–55.
  2. Jingchuan Luo et al., “Karyotype Engineering by Chromosome Fusion Leads to Reproductive Isolation in Yeast,” Nature 560, no. 7718 (2018): 392–96, doi: 10.1038/s41586-018-0374-x; Yangyang Shao et al., “Creating a Functional Single-Chromosome Yeast,” Nature 560, no. 7718 (2018): 331–35, doi: 10.1038/s41586-018-0382-x.
  3. J. A. Hackett, D. M. Feldser, and C. W. Greider, “Telomere Dysfunction Increases Mutation Rate and Genomic Instability,” Cell 106 (August 10, 2001): 275–86; Titia de Lange, “Shelterin: The Protein Complex That Shapes and Safeguards Human Telomeres,” Genes and Development 19, no. 18 (September 15, 2005): 2100–10; Paula Martínez and Maria A. Blasco, “Role of Shelterin in Cancer and Aging,” Aging Cell 9 (September 16, 2010): 653–66; Agnel Sfeir and Titia de Lange, “Removal of Shelterin Reveals the Telomere End-Protection Problem,” Science 336, no. 6081 (May 4, 2012): 593–97.
  4. Hugo Almeida and Miguel Godinho Ferreira, “Spontaneous Telomere to Telomere Fusions Occur in Unperturbed Fission Yeast Cells,” Nucleic Acids Research 41 no. 5 (March 1, 2013): 3056–67, doi: 10.1093/nar/gks1459.
  5. Alexander Etkind, “Beyond Eugenics: The Forgotten Scandal of Hybridizing Humans and Apes,” Studies in History and Philosophy of Biological and Biomedical Sciences 39 (July 2008): 205–10.
  6. Kirill Rossiianov, “Beyond Species: Il’ya Ivanov and His Experiments on Cross-Breeding Humans with Anthropoid Apes,” Science in Context 15, no. 2 (January 14, 2003): 277–316; S. Pain, “How to Breed a Model Citizen: The Forgotten Scandal of the Soviet Ape-Man,” New Scientist 199, no. 2670 (2008): 48–49.
  7. Shao et al., “Creating a Functional Single-Chromosome Yeast,” 331–35.

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