Problems With Dinosaur to Bird Evolution: The III Digits

One of the most widely held evolutionary claims in regards to the origin of species is the transition between Theropods and birds. Birds according to the evolutionary paradigm are considered to be the byproduct descendants from Theropod dinosaurs. The view that birds are the descendants from Theropods has been widely regarded as a scientific consensus. Just like how humans and their so-called closely related cousins the apes are asserted to have branch off from the last common ancestor, so too does the same concept apply for the transition between Theropods and early to modern day birds.

Proponents in favor of bird to dinosaur evolution have long relied in the fields of genetics, paleontology, comparative anatomy/embryology, to support this view. In addition, they hold these fields to be very strong evidence for their position in defense of dinosaur to bird evolution. In this article we are going to focus on one of the evidences that are claimed by these proponents to support this evolutionary view.

Evidence for Dinosaur to Bird Evolution: The III Fingered Digits

One of the evidence that is considered to be very strong evidence for dinosaur to bird evolution is the homology of the 3 fingered digits found between the hands of the advanced Triassic Theropods and in the forelimbs or wings that are found in the early and modern day birds. In the Triassic Theropod lineage, there were only 3 digits present in the theropod hands that were in the orders of Digits I, II, & III as demonstrated in the fossil record. Just like human hands, Digital I corresponded to the Thumb, Digit II for the index finger, and III for the middle finger. Because theropods in the Triassic period have lost Digits IV & V that were only present from their ancestors and birds are considered to have derived from the Theropod lineage that lost IV & V, it was predicted that the earlier to modern day birds should have the same evolutionary pathway of fingered digits. Theropods had Digits of I, II, & III and so the same homology should be true for birds if they truly derived from that lineage.

Studies in paleontology and comparative anatomy starting from the late 70s  investigating the developmental pathway of the fingers located in the bird wings/forelimbs have revealed that the fingered digits of the birds’ forelimbs do indeed follow the same pattern as dinosaurs. That is both the dinosaurs and birds follow Digits I, II, & III, exactly as predicted in the fossil record. Paleontologists have widely considered this as very strong evidence for dinosaur to bird evolution due to the strong homology present between them. This has been considered to be indisputable evidence that birds derived from the Triassic Theropod lineage. [1, 2, 3, 4]

The Conflict

Unfortunately, later studies in the field of embryology have challenged the common evolutionary view that the III digits present in birds provide very strong evidence for the dinosaur to bird transition. Contrary to the claims of paleontologists, embryologists have disputed the association of the I, II, & III digits homology as demonstrated in the fossil record and paleontological data. They claimed that the III digits in birds actually correspond to Digits II, III, & IV and not I, II, & III. The appearance of embryological data conflicting with that of paleontological have caused a great rage and controversy in regards to dinosaur to bird evolution. [5, 6, 7]

Although these two conflicting views were controversial, it was later confirmed by Alan in 2002 & Michael Richardson in 2003 using embryos that the development of bird forelimbs do indeed correspond to the digits of II, III, & IV unlike the Triassic Theropods of I, II, & III. By contrast, Digits I, II, & III that have been reported in the paleontology of birds were nothing more but the misleading result of convergent evolution. In other words, the III digits homology found between the dinosaurs and birds were due to different developmental pathways that converged independently to the same solution. Rather than those common characteristics being derived from common descent, birds acquired those digit similarities completely independent and distinct from the Triassic Theropod lineage. Thus, confirming the embryologists finding that the wing bird digits are II, III, & IV, not I, II, & III. [8, 9]

The Frameshift Mutation to the Rescue

Given the increasing evidence that the digit bird wings followed II, III, IV according to embryological papers and conflicted with the traditional I, II, III view predicted by the fossil record, the view that birds are dinosaurs was not dead yet. Some evolutionary biologists (despite others like Alan insisting that birds derived from an unknown lineage separate from Theropods) have insisted that such conflict between paleontology and embryology does not contradict the view of Theropods to birds transition. The reason, they claimed, is because of a frameshift that occurred during bird evolution. The Frameshift hypothesis predicts that the conflict of the birds possessing II, III, IV during embryogenesis can be best explained as the result of a frameshift mutation. This frameshift mutation is speculated to have occurred during bird evolution, where at that point the dinosaur digits of I, II, III have frameshifted into II, III, IV in the origin of bird evolution. Originally, on the onset action of bird evolution, the official digits were I, II, & III but since a frameshift has occurred, the birds digits shifted from I, II, III to II, III, & IV. A paper published in 1999 by J.P. & J.A. entitled, “A solution to the problem of the homology of the digits in the avian hand”.has been considered by some as a relief and savior to the conflicting view against dinosaur to bird evolution. [10]

Problems with the Frameshift Hypothesis

The frameshift hypothesis involves the introduction of complete homeosis, the transformation of one organ into another, arising from mutation in or misexpression of specific developmentally critical HOX genes. HOX genes are special protein-coding DNA segments that involve the protein synthesis of genetic switches. These proteins that act like on/off switches in the genome regulate the expression of other genes that play an important role in body-plan development by switching them on and off. HOX genes play a crucial in body-plan development, however, such body-plan structure is pre-determined by the cell long before the expression of HOX genes. Thus, any mutation in the HOX gene only affects the development of pre-determined body-plan structures. The Frameshift Hypothesis is considered as a resolution to the conflict between morphological and embryological data.

Molecular evidence in favor of the frameshift hypothesis by HOX gene expression is a 2004 paper published by Vargas & Fallon entitled, “Birds have dinosaur wings: the molecular evidence.” In this paper  Vargas & Fallon (2004) argue that the prospective digit 1 is characterized by the Hox d 13 expression alone while digits 2 to 5 have Hox d 13 and Hox d 12 expression. Since the anterior wing digit has only Hox d 13 gene expression, they argue it is digit 1 on the grounds of molecular homology, as quote unquote:

Our evidence contradicts the long standing argument that the development of the wing does not support the hypothesis that birds are living dinosaurs [11]

Unfortunately for these authors, their conclusion has been unconvincing as reported in the literature. A review paper published in the same year as Vargas & Fallon’s paper has criticized their claim that the solo presence of HOX D 13 expression only in the wing of birds has demonstrated unambiguously digits I, II & III. In their critique of Vargas & Fallon’s hypothesis Gallis (2005) argues convincingly that the molecular evidence cited is unconvincing as the mutants cited (eg talpid and Hox d deletion mutants) show only a weak correlation of Hox d 12/13 expression with digit identity. As quote unquote,

 

At the moment the data on mutant limbs does not present a challenge to the hypothesis that is based on developmental data; that is, the digits of bird wings are homologous to digits 2, 3, 4 in other amniotes. Other additional problems with the frameshift hypothesis are also highlighted in the same review paper as clearly demonstrated here.  [12,]

Although morphological data remain in conflict with that of embryological, not all embryological studies support the II, III, IV bird wing digit hypothesis. Fate mapping methodology based on Shh expression (Sonic Hedgehog Gene) and polarizing regions that relied on developmental/anatomical criteria have revealed that the digits of the birds are I, II, & III, exactly as predicted by the fossil record.

In Towers’ 2011 paper entitled, “Insights into bird wing evolution add digit specification from polarizing region fate maps” which the paper leads to the support of digits I, II, III & axis shift hypothesis, the authors have concluded that the three digits found in the forelimbs of neornithines developed from positions I, II & III. [13]

However, this view has remained inconsistent with the accepted early condensation patterns for digit progenitors, which show that the three hand digits of modern adult birds developed from the three middle positions. [14, 7, 9]. Furthermore, the assumption that a single frameshift mutation is capable of shifting Digits I, II III into II, III, IV has been shown to be unreliable given that such mutation reveals no adaptive advantage. Rather numerous frame shifts are required to even create an adaptive advantage, as demonstrated by Alan and other researchers. [15]

Another embryology paper that claims to support the Digits I, II, III view is a 2011 paper entitled, “Embryological Evidence Identifies Wing Digits in Birds as Digits I, II, and III. The methodology used was similar to Towers’ 2011 paper, and although they arrived to the same conclusion of Digits 1, 2, 3, it did not support the axis shift hypothesis.

However, just like Vargas & Fallon’s paper, this paper has been reviewed and criticized by a 2011 Nature paper entitled, “Comment on Embryological evidence identifies wing digits in birds as digits 1, 2, and 3.” Here, the authors critiquing the paper found the authors’ conclusions to be flawed and unconvincing due to incorrect methodology as highlighted here.

In summary, the peer-reviewed science literature shows no clear evidence that birds derived from Theropod dinosaurs. Studies in palenotology/comparitive anatomy criteria often tend to create contradictory results when compared to the data of embryology and positional criteria. The fact that these two fields don’t generally agree with each other has become rather a great difficulty for establishing dinosaur to bird evolution. In either case, both morphological and embryological analyses lead to the support that the bird digits are either I, II, III or II, III, IV. However, given the problems for the frameshift hypothesis, it has no longer been generally supported. Rather a new alternative hypothesis, known as the lateral shift, has been more supported in recent literature. However, lateral shift hypothesis unlike frameshift no longer predicts that the digits of the bird wings are I, II, III as predicted in the fossil record. The lateral shift hypothesis proposes that the digits of the wings are II, III, IV, showing no homology at all between the digits of Theropods and birds.

Given the contrary results, evolutionists can no longer claim using fingered digits homology as a back up. Rather than the literature demonstrating evidence of dinosaur to bird evolution, it has rather demonstrated quite the opposite.

References:

[1]   Gauthier, J. 1986. Saurischian monophyly and the origin of birds. Memoirs of the California Academy of Sciences, 8: 1-55.

[2] Ostrom, J. 1977. Archaeopteryx and the origin of birds. Biological Journal of the Linnean Society, 8: 91-182.

[3] Romer, A. S. 1966. Vertebrate Paleontology, University of Chicago Press, Chicago.

[4] Sereno, P.C. 1993. Shoulder girdles and forelimb of Herrerasaurus. Journal of Vertebrate Paleontology, 13: 425-450.

[5] Shubin, N. & Alberch, P. 1986. A morphogenetic approach to the origin and basic organisation of the tetrapod limb. Evolutionary Biology, 20: 319-387

[6] Muller, G.B.& Alberch, P. 1990. Ontogeny of the limb skeleton in Alligator mississipiensis: developmental invariance and change in the evolution of archosaur limbs. Journal of Morphology, 203:151-164.

[7] Burke, A. & Feduccia, A. 1997. Developmental patterns and the identification of the homologies in the avian hand. Science, 278: 666-669.

[8] Welten,.C.M., Verbeek, F.J., Meijer, A.H. & Richardson, M.K. 2005. Gene expression and digit homology in the chicken embryo wing. Evolution and Development, 7,18-28

[9] Nowicki, J. & Feduccia, A. 2002. The hand of birds revealed by early ostrich embryos. Naturwissenschaften, 89: 391-393.

[10] Wagner, G.P. & Gauthier, J.A. 1999. 1,2,3=2,3,4: a solution to the problem of the homology of the digits in the avian hand. Proceedings of the National Academy of Science USA, 96: 5111-5116.

[11] Vargas, A.O. & Fallon, J. 2004. Birds have dinosaur wings: the molecular evidence. Journal of Experimental Zoology (Molecular Development and Evolution), 304B, 85-89.

[12] Galis, F., Kundrat, M. & Metz, J. A. J. 2005. Hox genes, digit identities and the theropod/bird transition. Journal of Experimental Zoology (Molecular Development and Evolution), 304B: 198-205.

[13] Towers M, Signolet J, Sherman A, Sang H, Tickle C: Insights into bird wing evolution ad digit specification from polarizing region fate maps.

[14] “Embryological Evidence Identifies Wing Digits in Birds as Digits I, II, and III