by Paul Garner BSc (Hons), FGS
Introduction
The fossil record of horses has often featured in the scientific debate about origins, with many biologists regarding it as important evidence in support of evolutionary theory. For instance, in the textbook Biological Science, Soper (1997 p.890) says:
The horse provides one of the best examples of evolutionary history (phylogeny) based on an almost complete fossil record found in North American sedimentary deposits from the early Eocene to the present.
In this article, we will review the horse series as it is generally portrayed in textbooks and museum exhibits and consider its role as supportive evidence for evolutionary theory.
A brief description of the horse series
According to current thinking, the root of the family tree of the horse is to be found in a creature called Hyracotherium, whose fossils are known from the Lower Eocene of North America and Europe. Hyracotherium was a small mammal with four toes on the front feet and three on the rear. It had low-crowned teeth. Its characteristics are those of a forest-dwelling animal that browsed on foliage.
Mesohippus, a sheep-sized Oligocene form, had only three toes on the forefoot. As in Hyracotherium, the teeth were low-crowned. Mesohippus appears to have become extinct by the middle Oligocene, and is thought to have given rise to the characteristic Miocene genus Merychippus.
Merychippus also had three toes, but the central one apparently bore most of the weight. The structure of the foot suggests that a strong elastic ligament, like that of modern horses, passed behind this central toe. Unlike Hyracotherium and Mesohippus, the teeth of this horse were high-crowned, coated in cement, and had a more complex chewing surface.
It is thought that a branch from the Merychippus line led to Pliohippus, a late Miocene to Pliocene form. In Pliohippus the side toes became vestigial – although some species are now known to have had three toes. The teeth were high-crowned. A descendant of Pliohippus probably gave rise to the Pleistocene genus Equus, which rapidly spread to Europe, Asia, Africa, and South America. However, by the end of the Pleistocene the genus was extinct in the New World. Nine species survive to this day in the Old World – the wild Asian horse, four species of asses, and four zebras.
The major trends of horse evolution
It is common to find this story presented in diagrams illustrating what are considered to be the major trends of horse evolution (increased body size, reduction of side toes, increased height of teeth). In one sense, these trends are real. There is a trend towards the loss of toes from four on the forefoot and three on the hindfoot of Hyracotherium (4/3), to 3/3 in Mesohippus, and 1/1 in Equus. There is a trend towards increased height of teeth; all pre-Miocene horses were low-crowned, and crown heights increase from Merychippus to Equus. However, the textbook diagrams often make these trends look much more gradual and unidirectional than they really are in the fossil record.
Take, for instance, the increase in body size which is one of the most striking trends in popular presentations of the horse series. In fact, the increase in size between Hyracotherium and Equus was by no means gradual, constant, or progressive. An analysis by MacFadden (1987) of 24 ancestral-descendant species pairs revealed that 19 showed body-size increases. However, the remaining 5 lineages showed size decreases. There are even lineages within the horse family tree that show size decreases followed by reversals back to increased size (MacFadden 1984; Webb and Hulbert 1986; Hulbert 1988).
The real picture is more accurately portrayed in Figure 1, and even this oversimplifies the situation.
Figure 1. Interrelationships of the currently recognized genera of horses. After MacFadden (1992 p.99).
This diagram shows the horse family tree constructed by Bruce MacFadden, the foremost modern authority on fossil horses (MacFadden 1992 p.99). The family tree of the horse is more like a branching bush than a single straight trunk.
Popular presentations that suggest a simple, gradual, and progressive straight-line of evolution from Hyracotherium to Equus are not supported by the actual fossil data. Most evolutionary scientists now acknowledge that this is the case. For instance, Soper (1997 p.890), in Biological Science, writes:
The history of the horse does not show a gradual transition regularly spaced in time and locality, and neither is the fossil record totally complete.Similarly, in the textbook Advanced Biology, Roberts et al (2000 p.733) say:
...palaeontologists believe that there were numerous complications. For one thing, the rate at which evolution took place was probably not uniform, but sporadic and irregular. For another, there are thought to have been times when certain of the trends were reversed when, for instance, horses became smaller for a while.Evidence for evolutionary change?
Of course, none of this disproves the series; it is merely that textbooks and museum displays are simplistic, portraying only selected trends. They do not reflect all the twists, turns, offshoots, and dead-ends that are evident in the fossil record. However, non-evolutionary scientists do not need to disprove it – because, in fact, the horse series is perfectly consistent with a non-evolutionary explanation of the development of life.
There is a common misconception that scientists who reject evolutionary theory must believe that species are fixed and unchangeable. However, that is incorrect. Non-evolutionary scientists accept that species can change, but they believe that biological change has natural limits. Instead of the single evolutionary “tree of life”, according to which all living things have arisen from a single common ancestor, non-evolutionary scientists characterize the relationships between different living things as an orchard of trees (Figure 2).
Figure 2. The evolutionary tree of life represents the view that all organisms diverged from a common ancestor. By contrast, the non-evolutionist speaks of an orchard of life, where each basic type (e.g. horses, cats, dogs, humans) is represented by a different tree. After Wise (1990 p.358)
Each tree in the orchard represents a distinctly different group of organisms – what we might call a Basic Type – and each originated separately. In this non-evolutionary view, they cannot be traced back to a universal common ancestor. Nevertheless, each Basic Type is a broad group probably encompassing many species. While each Basic Type originated separately, a great deal of variation has occurred within the created group. For instance, all dogs – including wolves, coyotes, jackals, dingos and domestic dogs – probably belong to the same Basic Type. However, dogs are distinctly different from, and unrelated to, other groups (e.g. cats, bears, weasels).
Basic type biology, pioneered by Professor Siegfried Scherer and colleagues in Germany, seeks to identify the original Basic Types using hybridization (cross-breeding) studies (Scherer 1993). Scientists in the USA have suggested additional criteria for identifying and classifying the Basic Types, some of which can also be applied to fossil organisms (Wood and Murray 2003). This has developed into an exciting field of biological study with its own conferences and publications.
The Basic Type concept has been applied to horses, both living and extinct forms (Cavanaugh et al 2003; Garner 1998; Stein-Cadenbach 1993). These studies suggest that all horses, including the 150 or so fossil species, are probably related in a single Basic Type. The ancestor(s) of these horses probably possessed latent (i.e. unexpressed) genetic information that gave the horse type tremendous potential for variety. One way in which this latent genetic potential may be regulated is by differential gene expression. By this we mean that in living organisms there are mechanisms by which genes can be turned on (i.e., expressed) or turned off (i.e., not expressed). For example, horses may have a genetic ‘switch’ that determines whether they develop side toes. Other regulatory genes may control size, shape of the teeth, and so on.
Consistent with this theory, there is evidence for differential gene expression in modern horses. Ewart (1894) studied horse embryos and found that at an early stage of development tiny limb buds appeared beneath the splint bones. When he dissected these buds to determine their internal structure he found that they resembled toes, with caps at the end that he believed represented hooves. At the most advanced stage it was even possible to distinguish three individual elements in the buds, corresponding to the three bones in the side toes of Merychippus. After this, and prior to birth, the limb buds were lost. It appears that modern horses retain the genetic potential for extra toes, but that regulatory genes switch off the structural genes for side toes during embryological development. Occasionally, something goes awry with this regulatory mechanism and foals are born with side toes (e.g., Marsh 1879, 1892; Struthers 1893).
Summary
The evidence of fossils, along with the study of horse embryos, indicates that the horse series is a genuine record of biological change over time. Evolutionary scientists point to this as evidence of Darwinian evolution. However, non-evolutionary scientists say that this simply records changes within the horse basic type and that there is little evidence to suggest that horses developed from a non-horse ancestor. Since the magnitude and type of change represented by the horse series can be accommodated by both evolutionary and non-evolutionary theories it cannot, therefore, distinguish between them. At best, in terms of the origins debate, the horse series is neutral data.
References
Cavanaugh, D.P., Wood, T.C., Wise, K.P. 2003. Fossil Equidae: a monobaraminic, stratomorphic series, in: Ivey, R.L., editor. Proceedings of the Fifth International Conference on Creationism. Creation Science Fellowship, Pittsburgh, pp.143-153.
Ewart, J.C. 1894. The development of the skeleton of the limbs of the horse, with observations on polydactyly. Journal of Anatomy and Physiology 28:342-69.
Garner P. 1998. It’s a horse, of course! A creationist view of phylogenetic change in the equid family. Origins (25):13-23.
Hulbert, R.C. 1988. Calippus and Protohippus (Mammalia, Perissodactyla, Equidae) from the Miocene (Barstovian-early Hemphillian) of the Gulf Coastal Plain. Bulletin of the Florida State Museum of Biological Sciences 32:221-340.
MacFadden, B.J. 1984. Systematics and phylogeny of Hipparion, Neohipparion, Nannippus, and Cormohipparion (Mammalia, Equidae) from the Miocene and Pliocene of the New World. Bulletin of the American Museum of Natural History179:1-196.
MacFadden, B.J. 1987. Fossil horses from ‘Eohippus’ (Hyracotherium) to Equus: scaling, Cope’s law, and the evolution of body size. Paleobiology 12:355-69.
MacFadden, B.J. 1992. Fossil horses: systematics, paleobiology, and evolution of the family Equidae. Cambridge University Press, Cambridge.
Marsh, O.C. 1879. Polydactyle horses, recent and extinct. American Journal of Science17:499-505.
Marsh, O.C. 1892. Recent polydactyle horses. American Journal of Science 43:339-55.
Roberts, M., Reiss, M., Monger, G. 2000. Advanced Biology. Nelson.
Scherer, S., editor. 1993. Typen des Lebens. Pascal-Verlag, Berlin. [German language publication]
Soper, R., editor. 1997. Biological Science 1 and 2. Third Edition. Cambridge University Press, Cambridge.
Stein-Cadenbach, H. 2003. Hybriden, Chromosomenstrukturen und Artbildung bei Pferden (Equidae), in: Scherer, S., editor. Typen des Lebens. Pascal-Verlag, Berlin, pp.225-244. [German language publication]
Struthers, J. 1893. On the development of the bones of the foot of the horse, and of digital bones generally and on a case of polydactyly in the horse. Journal of Anatomy and Physiology 28:51-62.
Webb, S.D., Hulbert, R.C.. 1986. Systematics and evolution of Pseudhipparion(Mammalia, Equidae) from the late Neogene of the Gulf Coastal Plain and the Great Plains, in: Flanagan, K.M., Lillegraven, J.A., editors. Vertebrates, phylogeny, and philosophy. Contributions to Geology, University of Wyoming, Special Paper 3.
Wise, K.P. 1990. Baraminology: a young-earth creation biosystematic method, in: Walsh, R.E., Brooks, C.L., editors. Proceedings of the Second International Conference on Creationism: Volume II. Creation Science Fellowship, Pittsburgh, pp.345-358.
Wood, T.C., Murray, M.J. 2003. Understanding the Pattern of Life: Origins and Organization of the Species. Broadman and Holman Publishers, Nashville, Tennessee.
© Paul Garner, 2005.