Friday, 13 April 2018

Who We Are and How We Got Here

Oxford University Press 2018
ISBN 978-0-19-882125-0
In little more than a decade, studies of ancient DNA have transformed our knowledge of human history. A recurrent theme is the migration of people over long distances to mingle with and often to replace existing populations. Often the immigrants brought with them new technologies and languages. Although there are also examples of skills dispersing without migration (spread of the Beaker culture from Iberia) and succesive waves of migrants adopting local languages (southwest Pacific islands).

All this and much more is related in David Reich's new book. Although his prose is stilted at times, that is more than compensated by the breadth of material presented and by excellent diagrams. These include maps showing the probable origin, direction and timing of human migrations. 

Skull from Lapa Vermelha, Lagōa Santa, Brazil
Photo by Ryan Somma CC BY-SA 2.0
Although Reich delights in disproving established concepts, it is interesting to find that some long-standing ideas receive support. For example, there is a clear genomic evidence of two waves of migration into Central and Southern America. The first wave of migrants designated "Population Y" left a genetic signature that is strongest in some isolated Brazilian communities (Tupí). This finding lends support to the concept of an ancient people called Palaeoamericans by Neves (here) and named a century ago as Palaeo-Amerind by Haddon (previous post). These ideas were fostered by excavations at Lagõa Santa, Brazil, started 175 years ago by the Danish scientist Peter Lund and continued until this day.

Similarly, present day peoples of India derive much of their genetic makeup from two previous populations. One of these described as "Ancestral South Asians" bears a striking resemblance to the "Pre-Dravidians" postulated by Haddon.

There has been some animus engendered by Reich's book - exacerbated by an opinion piece he wrote for The New York Times. Part II of the book has chapters on the origins of Europeans, South Asians, Native Americans, East Asians and Africans. Inevitably this division reflects the broad racial categories of the past. The flight from racial stereotyping in the last 50 years is laudable, but a mist of political correctness can make recent studies hard to decipher. Reich's book disperses the mist and inevitably invites controversy. 

Wednesday, 28 March 2018

IFPA now on Twitter

The International Federation of Placenta Associations (IFPA) now has a Twitter account: @IFPA_Official 

Abstract deadline for the upcoming meeting i Tokyo is 2 April 2018.

Monday, 26 March 2018

Parturition and placentophagy in baboons

Yellow Baboon (Papio cynocephalus) female with young
Wikimedia Commons
Two important papers just appeared on reproduction in baboons (Papio spp.). Natalia Schlabritz-Loutsevitch and co-authors (OA here) describe normal birth and pregnancy complications as observed in the laboratory. Their study was based on imaging (MRI, X-ray, ultrasound) and video recording of deliveries. 

I found it particularly fascinating to see the delivery process. When the baby's head emerges it is face upwards so looking directly at the mother (unlike in humans and chimpanzees where the head emerges face down). After cleaning the baby, the mother ignores it for several minutes and devotes all her attention to eating the placenta. Some of this can be seen in the delivery video (supplementary material here). 

Gesquiere and co-authors (here) summarize no less than 36 years of observations in free-ranging baboons, mainly yellow baboons (P. cynocephalus) from Amboseli in southern Kenya. Their focus is on what determines the interval between births. The main determinant was lactational amenorrhoea due to suppression of the ovarian cycle while the infant was breastfeeding. Once the baby was weaned, most females became pregnant again within 6-8 cycles.

There is an interesting parallel with human hunter-gatherers (previous post). Based on data from the !Kung people of South Africa, Roger Short (here) calculated that a woman in a hunter-gatherer society was pregnant for nearly four years of her life and spent 15 years in lactational amenorrhoea. As a consequence she endured menstrual cycles for rather less than four years. Short contrasted this with present day society where a woman might expect to have menstrual cycles for 35 years of her reproductive life.

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.