Friday, 23 February 2018

Przewalski's horse an also-ran

Przewalski's horse Photo by Claudia Feh
CC BY-SA 4.0 
Przewalski's horse has long been seen as the last population of wild horses. No more. A new attempt to find the origins of domestication using ancient DNA is full of surprises (here). 

The Botai Culture of Central Asia has been tied to domestication of horses some 5500 years ago. Ancient DNA was extracted from the bones of Botai horses and the genomes compared with those of other ancient and modern horses.

Cord at term of Przewalski's horse
From Benirschke Comparative Placentation
The first surprise was that Przewalski's horse clustered with the Botai horses. The inescapable conclusion is that Przewalski's is a feral population descended from those domesticated at Botai.

The second finding was that all other horses descend from a separate branch. This was foreshadowed in a previous study (previous post). The most parsimonious explanation is that there was a second centre of domestication yet to be identified.

Benirschke has several images of the placenta of Przewalski's horse (see also previous post).

Sunday, 18 February 2018

Mammal Diversity Database

Zebu cattle (Bos taurus indicus)
Agricultural Research Service (USDA) Public Domain
How many species of mammals are there? That is the appropriate title of an article (OA here) accompanying the launch of the American Society of Mammalogy's new Mammal Diversity Database (here).

The answer is 6495 species and counting. Compared to Wilson & Reeder Mammal Species of the World (MSW3), which has its own database (here), that is an increase of 1250 in 13 years.

The authors of ASM MDD have erred in favour of the splitters. In particular, acceptance of the taxonomy of Groves & Grubb for Artiodactyla and Perissodactyla (previous post) will raise eyebrows. In anticipation they write, "inclusion of the taxonomy of Groves and Grubb (2011) in the MDD ensures that these taxa are vetted by the greater mammalogical community."

I note, however, that unlike Castello (previous post), they list the zebu as a subspecies of Bos taurus. Important because this species is a focus in research on bovine placentation and reproduction in places like Brazil.

I found the new MDD more difficult to navigate than MSW3. For example, if you enter a higher taxon than genus you get all the entries in that taxon rather than any definition of the Family or Order itself.

As far as I can see there is no possibility to search for synonyms as on MSW3. I have found this search function of MSW3 enormously valuable when delving into the older literature on placentation. Similarly there is little information about subspecies names.

Some choices of terminology meet my approval. They drop Cetartiodactyla in favour of Artiodactyla (whales and their kin are nested within even toed ungulates). Inevitably they retain Afrosoricida for the order that includes tenrecs, otter shrews and golden moles (a really unfortunate name as African shrews are numerous and belong to an entirely different clade).

Most regrettaby, however, they retain Infraclass Placentalia for eutherians. This unfortunate term was introduced by McKenna & Bell and I have ranted about it before (previous post). It has caused countless confusion and the vernacular "placentals" or even "placental mammals" can mislead the unwary to believe marsupials are without placentation. Prothero among them (previous post).

Thursday, 8 February 2018

Placentation in the wildebeest

Blue Wildebeest (Connochaetes taurinus)
Photo by Muhammad Mahdi Karim (Wikimedia Commons)
In ruminants binucleate trophoblast cells (BNCs) migrate to and fuse with uterine epithelial cells to form a fetomaternal hybrid - either a syncytium or trinucleate cells. This remarkable mechanism was described some 40 years ago by Peter Wooding. The fusion is aided by a syncytin, the product of an endogenous retroviral gene (previous post).

Almost all the ruminants studied hitherto have trinucleate cells. A fetomaternal syncytium is formed in the basal tragulids (chevrotains), which have a diffuse placenta without cotyledons. The other exception hitherto is syncytium formation in sheep and goats.

Binucleate trophoblast cell of bovine placenta from Benirschke
Now Wooding et al. (here) have undertaken to survey a wide range of ruminants including a chevrotain (Tragulidae), 8 bovids (Bovidae), 8 deer(Cervidae), the pronghorn (Antilocapridae) and a giraffe (Giraffidae). Only the musk deer (Moschidae) are missing.

Almost all the pecoran ruminants studied had trinucleate cells. Exceptions were the sheep and the wildebeest (Connochaetes taurinus). This is a new and highly interesting observation.

Three groups of bovids, classified as Tribes by Groves and Grubb (previous post) and Subfamilies by Wilson and Reeder share a common ancester (here and here). These are Alcelaphini, Hippotragini and Caprini. The first includes the wildebeest and the last sheep and goat. So it is likely that the most recent common ancestor (MRCA) of sheep and goats and the wildebeest had a fetomaternal syncytrium.  

To summarize. The basal Tragulidae have fusion of BNCs and maternal epithelium to form a syncytium. The trinucleate cell replaced this in the MRCA of pecoran ruminants (those with cotyledons). Then a fetomaternal syncytium reappeared in the MRCA of wildebeest and sheep and goats. 

To test this hypothesis it would be useful to have studies of the third tribe Hippotragini, i.e. an oryx, the roan and sable antelopes or the bluebuck.

Tuesday, 6 February 2018

Our Human Story - Book Review

ISBN 978 0 565 09391 4 Cover Price £14.99
This short book (160 pages) can be highly recommended to anyone who, like me, often loses the plot when reading papers on human evolution. It covers all phases in hominin evolution during the past 7 million years. The most important fossils are named (and the names explained), illustrated, and placed in geographical context by some excellent maps.

There is a good index so next time you are unsure what is referred to by KNM-ER 1470 you can quickly look it up!

I had anticipated that the book might lack the very latest info such as the redating of the early humans from Jebel Irhoud or the early dispersals to Arabia and India. But Louise Humphrey and Chris Springer have it all. Only the 120,000 year-old human fossil from Misliya, Israel (here) was published too late to make it into their book.

Chris Stringer is a strong proponent of the Out of Africa hypothesis that has all modern populations deriving from a single dispersal 45-55,000 years ago. But in this book alternative scenarios are given a fair treatment.

Near the end there is a neat diagram summarizing human evolution during the last million years.

Monday, 29 January 2018

Evolution of brain shape - and why babies are not better developed at birth

Brain shape of present-day human (left) and a Neanderthal (right)
From Neubauer, Hublin and Gunz (OA here) CC BY-NC
The difference between Neanderthals and modern humans lies not so much in brain size as in brain shape. In modern humans, the brain changes from an elongated to a more globular shape. This occurs mainly after birth (OA here). 

Now a remarkable paper in Science Advances (OA here) shows evolution of brain shape was a gradual process in Homo sapiens. The earliest modern humans from North and East Africa (300,000 to 200,000 years ago) had much more elongated brains.

Brain shape comparable witrh modern humans is first seen in fossil skulls that are 35,000 to 10,000 years old. These are associated with the more advanced culture of the Upper Paleolithic, i.e. with behavioural modernity.

Conveniently, there is a group of fossils from 130,000 to 100,000 years ago with intermediate brain shapes. They include some from the Levant mentioned in my last post (here). It has been suggested that they represent an early dispersal from Africa that reached far into Asia and persisted for a long period of time. Perhaps they were supplanted by a second wave of humans with more globular brains and a more advanced culture.

Quite a lot has been written about the constraints on human childbirth consequent on evolution of a larger brain (e.g. OA here and here). Changing the shape of the brain and skull could have complicated matters further. Maybe that is why most of the rounding up of the brain, which includes rapid growth of the cerebellum, occurs in the first months of life. The other region that rounds up is the parietal area. It is thought that this reflects changes in "an important hub of brain organization" called the precuneus.

Most mammals with a long gestation and a single young have newborns that are well developed (previous post). Babies need extensíve parental care leading Portmann to describe humans as "secondarily altricial." Maybe the postnatal growth of critical brain areas, reflected in a more globular brain, is part of the explanation for this conundrum. 

P.S. Right now there is a great deal of buzz about a modern human fossil from Israel dated to 177,000 years ago (OA here). As there is no cranium it cannot yield data on brain size.

Wednesday, 24 January 2018

Did humans reach Asia earlier than we think?

Map of early human migration
NordNordWest (public domain)
An insightful perspective in Nature Ecology and Evolution (abstract here) questions if there was just one successful dispersal of Homo sapiens from Africa to Asia. The current consensus - the Out of Africa Hypothesis (here) - is that modern humans left Africa some 65-55 thousand years ago and rapidly spread around the globe (see map). 
Stone hand axe from Wadi Dabsa, Saudi Arabia
From Foulds et al. (here)
An earlier migration from Africa to the Levant is supported by fossils and increasingly by archeological evidence, including from recent digs in the Arabian peninsular (here). Hitherto this exodus has been regarded as short lived and restricted in geographical extent.

Leveraging the increasing amount of evidence emerging from Asia, Rabett argues that, on the contrary, this first dispersal reached far beyond the Levant with populations surviving for many thousands of years.

Therefore he questions whether all the sites attributed to Homo sapiens in Southeast Asia and Australia stem from a single dispersal. Some sites from 45-65 thousand years ago might represent late-surviving enclaves of an earlier dispersal. At the very least, much of the archeologcal data conflicts with a rapid dispersal along the Southern coastal route.

Genomic data including ancient DNA support the view that all human populations living outside Africa derive from a single exodus albeit there was subsequent admixture with other hominins (including Neanderthals and Denisovans). The earlier migration did not contribute to the extant human gene pool. But to quote Rabett, "should evolutionary success be measured only by genetic and demographic continuity into the present?"

Friday, 12 January 2018

Temerity of the tenrec

Lesser hedgehog tenrec © Peter J Stephenson
After human and mouse the tenrec was one of the first mammals to be sequenced (link here). When this was still in the works, the popular science magazine ScienceNews put a tenrec on the cover. The caption to the article read, "They're sequencing a what?" (here). 

Sadly the tenrec has had it's 15 minutes of fame. When Nature News and Comment ran a report on a recent study from the Chavan lab., it was headlined, "Armadillo, hedgehog and rabbit genes reveal how pregnancy evolved." Once the editors realized Echinops telfairi was a tenrec, this was shortened to "Armadillo and rabbit genes..."

The irony to this was that a previous paper from the lab. (OA here) wrote that their hypothesis - about implantation evolving from an inflammatory reaction - could be tested by looking at the basal eutherian clades Afrotheria and Xenarthra. The lecture on which Nature News and Comment was reporting showed this had now been addressed in a tenrec (Afrotheria) and an armadillo (Xenarthra).

The temerity of a tenrec dressing up like a hedgehog!