Friday, 23 February 2018

A feral ancestry for the Przewalski's horse

Sometimes timing is funny. In November 2017 I wrote a post defending the Przewalski's horse's status as a wild animal. A new study by Orlando et al. has challenged the status of the Przewalski's horse as the last living genuine wild horse.
The earliest archaeological evidence of horse husbandry is from the Botai culture of Kazakstan from 5.500 years ago. It has been assumed previously that these Botai horses belong to the earliest strain of domestic horses of the caballine lineage. Surprisingly, the authors found only about 2,7% Botai-related ancestry for all domestic horses from 4.000 years ago, while the authors claim the Botai horses turned out to be the ancestral stock of the modern Przewalski's horses population. I have not read the paper yet because it is behind a paywall, thus I cannot see what led the authors to the conclusion that all modern Przewalski's horses descend from the Botai population and not just that the Botai population was part of the przewalskii clade. But let us assume the former is the case for now.
The authors thus write that Przewalski's horses are feral descendants of these early domestic horses, and not true wild horses. I have problems with the latter part of the conclusion. First of all, the horses of the Botai culture must have been in a very early state of domestication, and not for a long time. After escaping human husbandry, those early domestic horses have been exposed to natural selection for four millennia again. Any changes that might have occurred in the early state of domestication must have adapted to the requirements of living as a wild animal again. Thus, I don't regard the short (in evolutionary terms) episode of domestication as substantial enough to categorize the Przewalski's horse as feral instead of a wild animal. In fact, ever since its discovery, the Przewalski's horse has been used as a model for a wild equine and numerous differences in physiology, development and behaviour have been noted between the Przewalski's horse and domestic caballine horses as much as feral caballine horses. Przewalski's horses are way less tamable and more aggressive than domestic caballine horses, and genomic studies have shown that numerous genes for physiological aspects have been altered through human utilization while that is not the case in the Przewalki's horse[1]. The recent study by Orlando et al., or to be precise the short period of early domestication, does not alter this fact. The situation is not comparable to mustangs and other feral caballine populations at al.
Thus I do not regard the Przewalski's horse as any less wild as before. One could use it as an example for a "post-domestic wildtype", a term I tried to introduce in my posts on dedomestication as opposed to "pre-domestic", a term that is already used. However, I am rather confident that the majority of authors will begin to list the Przewalski's horse as a feral horse now, as the 19th century conception of  nature and evolution as something static and humans as an irreversible altering factor is still prevalent in a lot of experts heads. Unfortunately, in my opinion.

Interestingly, the authors found the alleles for the Leopard spotted colour present in the Botai horses. This colour variant has already been found in Pleistocene wild horses (see earlier studies of Orlando et al.).

Literature

Orlando et al. 2018: Ancient genomes revisit the ancestry of domestic and Przewalski's horses.


[1] Schubert et al.: Prehistoric genomes reveal the genetic foundation and costs of horse domestication. 2014.

14 comments:

  1. Hi Daniel, I came across this today and it's a bit surprising. I don't have any knowledge on the science of this but I would have a few questions on the circumstances of horse domestication. From the findings of this paper the initial domestication was of the Przewalski line and subsequently the caballine line predominates in the domestic horse gene pool. I would guess from this that both populations were relatively common in the area where domestication occurred and not just the last few of there kind. I would think that the Przewalski line continued to the East and caballine to the West (I am probably wrong here). I find it hard to believe that the feral decendants of the first domestic horses could overwhelm the existing wild Przewalski population. But many things could have happened; maybe the Botai people could relatively quickly have hunted out the remaining wild horses opening a niche for the feral horses.Is there a continuous fossil record of Przewalski horses from the last Ice Age until now in Mongolia? I don't know the answer. I guess this whole subject will be clarified in the next few years.

    ReplyDelete
    Replies
    1. The main problem is that horses are often indistinguishable or barely distinguishable based on bone material alone, which is why the Botai horses have been considered of caballine origin.

      Delete
  2. This comment has been removed by the author.

    ReplyDelete
  3. http://sci-hub.la/http://science.sciencemag.org/content/early/2018/02/21/science.aao3297?rss=1

    To get articles sci-hub.la helps.

    ReplyDelete
    Replies
    1. Ha ! I've just created a free AAAS-account to read this, so this wasn't necassary.
      However, the article is not behind a paywall...

      Delete
  4. Hi again Daniel, while I am still trying to absorb the above bombshell, have you seen the recent paper by Michael Hofreiter and co.; "A comprehensive genomic history of extinct and living elephants"?published recently? I have just seen an abstract but it looks like the Straight-tusked Elephant had a more complex history that the study two years ago indicated with Woolly Mammoth contributing to its genome. I would only guess that this would be related to these species coexisting and interbreeding in a restricted area in Southern Europe during the height of variou glacials possibly similar to the Aurochs ancestry recently discovered in Wisents. Anyway, I am only guessing and it will be explained by others shortly no doubt.

    ReplyDelete
    Replies
    1. I haven't read the paper yet, but I know the results, also mentioned them in my recent post on hybridization. I think that neighbouring related species nearly always interbreed; in each animal groups where it was checked they discovered signs of mutual hybridization so it seems to be pretty common.

      Delete
    2. Indeed you did, I should have checked! It passed my mind that it was coincidental that you had written a post about hybridisation recently.

      Delete
  5. Hallo,
    Ich wollte nur einmal sagen, dass ich ihren Blog sehr gerne mag und jedes mal interessiert lese. Ich wollte mal fragen ob Sie vielleicht die jüngere Evolutionsgeschichte von Bos,Bison, Poephagus erklären könnten. Ich bin etwas verwirrt, wie sich die Arten entwickelt haben und in welchem Zusammenhang Leptobos und Pelorovis damit stehen und wo der geographische Ursprung der Gattungen ist. Ich habe auch gelesen das Bos acutifrons der Vorfahre der heutigen Bos Arten ist. Gilt dies für den Auerochsen, Banteng, Gaur, Kouprey und den Yak? Das wird mir noch nicht so klar. Es würde mich freuen wenn Sie darüber schreiben könnten. Mit freundlichen Grüßen
    Johannes Süß

    ReplyDelete
    Replies
    1. Hallo, die Gattungen Bison und Bos werden jeweils auf Leptobos und Pelorovis zurückgeführt. Bos acutifrons wird oft als der Vorfahre von Auerochse und anderen Bos-Rindren geführt, allerdings liegt deren Ursprung wohl in Afrika. Meine Vermutung ist dass B. acutifrons und B. buaiensis eine langhörnige Bos-Klade gebildet haben, die nicht die direkten Vorfahren der heutigen Bos-arten darstellen. So wie es aussieht sind Yaks phylogenetisch näher zu Bisons als zu Rindern im engeren Sinne, und Wisente dürften wiederum durch Hybridisierung von Bisons und Auerochsen entstanden sein. Diese Tatsache, sowie dass Bisons, Yaks und Rinder i.e.S. eingeschränkt kreuzbar sind, bringt heute viele Autoren dazu die Gattungen Bison, Bos und Poephagus als eine Bos, zusammenzufassen. Dies würde bedeuten, dass auch Pelorovis und Leptobos unter Bos fallen, da auch diese mit jenen kreuzbar gewesen wären, und dass Bos eine sehr große, phänotypisch sehr variable Gattung wäre. Ich tue das nicht, ich "splitte" lieber und differenziere zwischen Bos, Bison und Poephagus. Taxonomie ist nicht konsistent sondern subjektiv und unterliegt Konventionen & Traditionen. Da gibt es viele Beispiele.

      Delete
    2. Danke, das hat mir schonmal sehr weitergeholfen. Das Taxonomoie sehr subjektiv ist, ist mir schon oft aufgefallen. Ich bevorzuge alle in bos zusammenzufassen. Ich finde das Yak besonders interessant, da es ja äußerlich bos prigemius sehr ähnlich sieht und auch die Hornform identisch ist. Gibt es Hinweise darauf, dass in der Evolutionsgeschichte auch Hybridisierung eine Rolle gespielt hat?

      Delete
    3. Die Ähnlichkeit zwischen der Hornform von Yak und Auerochse halte ich eher für Konvergenz; diese Primigenius-Form findet man bei vielen Arten, wahrscheinlich weil sie zweckmäßig ist den Gegner herumzuziehen, wegzudrücken und sich einzuhaken. Man findet sie sogar bei den Stoßzähnen der Mammuts, die ja ähnlich kämpften, nur auf den Kopf gestellt.

      Delete
  6. According to recent genetic research the European bison appears to be a hybrid between Bos primigenius and the Woodland bison, Bison Shoetensacki, rather than B.priscus.

    ReplyDelete
    Replies
    1. Indeed, the paper came out a few weeks after the post.

      Delete