Defining the genus Homo

Reconstruction of Homo erectus georgicus by Élisabeth Daynes
Licensed under CC BY-SA 3.0 via Wikipedia Commons

A thought provoking essay in the current issue of Science (here) asserts definitions of the genus Homo and the species assigned to it remain "as fuzzy as ever." They drive home their point by showing, on the one hand, specimens attributed to Homo with australopithecine features and, on the other, australopithecine fossils with features hitherto claimed to be confined to Homo.

This is a brief but instructive read. The take home message is that hominid systematics needs to be rethought. In the process we may need to scrap "the iconic list of names in which fossil specimens have historically been trapped."

Cover of Third Edition 1956

One name that may have to go is Homo habilis (previous post), the handy man. Louis Leakey, it is suggested, was keen (perhaps too keen) to identify this fossil as the maker of simple tools following the dictum of Man the Toolmaker. It was a pleasant surprise to find Kenneth P. Oakley's booklet cited as an influence on previous and current thinking. My well thumbed copy dates from my schooldays.

Viviparity in extinct reptiles

Mesosaur - an aquatic reptile from the Palaeozoic
Nobu Tamura (CC BY 2.5 via Wikimedia Commons)

Dan Blackburn and Christian Sidor have written an interesting paper (here)summarizing the fossil evidence for viviparity in the Late Palaeozoic and Mesozoic Eras. They document a minimum of six separate evolutionary origins of viviparity. Most are aquatic reptiles including mesosaurs (picture), sauropterygians, ichthyopterygians, choristoderans and mosasauroids. The sixth example is a Cretaceous lizard Yabeinosaurus.



Mosasaurus hoffmannii - an aquatic reptile from the Late Cretaceous
 Nobu Tamura (CC BY 3.0 via Wikimedia Commons)

 It cannot be established with certainty that the fetuses were supported by a placenta, since soft tissues usually do not fossilize. The probability is great, however, given the advanced stage of development at birth and by analogy with present day snakes and lizards. Some have extremely complex placentas an example being the New World lizard Mabuya (here).

Materpiscis attenboroughi - a live-bearing fish from the Late Devonian 
Sularko - Museum Victoria (Licensed under CC BY-SA 3.0 via Wikimedia Commons)


My own article on fossil evidence for viviparity (here) was prompted by the discovery of an extremely well preserved fish with a putative umbilical cord (here).

The chicken or the egg

 As an afterthought it is of interest that the original amniote egg had a leathery shell like many reptiles today. A calcified shell was evolved at least four times (here) including in the dinosaur lineage that led to the birds. The calcified egg came before the chicken!    

Human evolution: brain, birthweight and the immune system

The Royal Society recently published a special issue of Philosophical Transactions B entitled Human evolution: brain, birthweight and the immune system, edited by Graham J Burton, Ashley Moffett and Barry Keverne. The interesting theme is summarized as follows:

"The complexity of the human brain is unique amongst mammals. However, the large size of the brain at birth poses risks to mother and offspring due to constraints on pelvic architecture and uterine perfusion imposed by bipedalism, the so-called ‘obstetric dilemma’. The growth of the human brain in utero is believed to be permitted by our unique highly invasive form of placentation. The structure of the placenta varies to a greater extent between different mammalian species than any other organ. In particular, there is a spectrum of invasiveness of the fetal tissues into the wall of the uterus, even amongst the apes. Invasion poses unique immunological challenges as the migrating trophoblast cells expressing paternal genes intermingle with cells of the maternal immune system in the uterine wall that must be negotiated. Recent advances have also revealed important insights into the genetic and epigenetic links between the regulation of placental and brain growth, centred on imprinted genes that are expressed in a parent-of-origin manner. This complex network of interactions regulating brain development is explored in this Theme Issue in the light of new concepts in placental evolution, the immune system at the maternal-fetal interface, and genomic imprinting."

The table of contents can be found here. Disclosure: I co-authored one of the papers.

South American fossil ungulates are related to modern horses

Macrauchenia - an extinct South American native ungulate
Drawn by Kobrina Olga (Wikimedia Commons CC BY 3.0)

South American mammals either arrived recently during the Great American Interchange, came to South America by rafting (previous post), or have a shallow fossil record (xenarthrans). In contrast, South American native ungulates (SANU) have a deep fossil record. Though the lineage is now extinct, genera such as Macrauchenia (above) and Toxodon survived into the Late Quaternary. The conditions under which these fossils were conserved did not favour survival of their DNA. However, a paper just out in Nature (here) shows how proteomics can be used to probe their phylogenetic affinities.

The authors used MALDI-TOF mass spectrometry to determine the amino acid sequences of the alpha chains of type I collagen extracted from fossil bones. They then performed a phylogenetic analysis. This showed that Macrauchenia and Toxodon form a monophyletic group. Interestingly, this resolved as the sister group to modern Perissodactyla, i.e. horses, tapirs and rhinoceroses. Together they form a taxon named Panperissodactyla.

The modern species have an epitheliochorial placenta so perhaps this type was present in the sister species Macrauchenia  and Toxodon.

Placentalia - a subgroup of placental mammals

Sir Richard Owen (1804-1892)

The current literature abounds with the term "placental mammals;" it often appears in a paper's title. Even though marsupials have a yolk sac placenta - and several a chorioallantoic placenta - they are not included in the definition of Placentalia. This is a constant source of irritation to marsupial biologists and a pitfall for those wishing to stay their friends (see between Scylla and Charibdis).

Placentalia was relaunched by McKenna and Bell in their influential volume Classification of Mammals above the Species Level (New York 1997). It is attributed to Sir Richard Owen (pictured above).

Owen 1837 [sic] page 903

This is the passage cited by McKenna and Bell. It may well be the first usage of Placentalia but is far from being a definition.

In its current usage, Placentalia defines a crown group of mammals comprising extant species and their ancestors. It is not the same as Eutheria, which includes lineages that do not have living descendants. Placentalia therefore is popular in molecular phylogenetics and phylogenomics. The reason is evident: when working with DNA sequences from living species, inferences can be drawn only about character evolution in the crown group.

When looking for the Owen reference I found that McKenna and Bell had got the date wrong. Robert B. Todd commenced publication of The Cyclopaedia of Anatomy and Physiology in 1837 but Owen's article is in volume 4 part 2 published 1849-52 (full reference below).

Owen R. Teeth. In Todd R.B. (Ed.) The Cyclopaedia of Anatomy and Physiology Vol. 4 Part 2, pp. 864-935. London: Longman, Brown, Green, Longmans and Roberts, 1849-1852. McKenna and Bell Refer to page 903.

Embryologists then and now

International Institute of Embryology London 2-5 August 1938

The photos in this post are separated by three quarters of a century. We know the identities of the 1938 embryologists because an annotated copy was sent by Fritz Strauss to Harland W. Mossman and is curated in the latter's collection in Madison, Wisconsin.

Top row (left to right): T. Thomson Flynn (Belfast); H. M. W. Woerdeman (Amsterdam); Hans Bluntschli (Bern); Jan Florian (Brno).

Middle row: G.L. Streeter (Carnegie Institution); Karl Peter (Greifswald); Mrs. Katherine Jones Hill (London); E.S. Goodrich (Oxford); Miss E. G. Fraser (London); Warren H. Lewis (Carnegie Institution); A. Celestino da Costa (Lisbon); Paul Gérard (Brussels); H. Woollard (London). 

Bottom row: Otto Grosser (Prague); J.T. Wilson (Cambridge); J. Boeke (Utrecht); Honor B. Fell (Cambridge); Dan de Lange (Utrecht); J. P. Hill (London).

It was my pleasure to attend a recent meeting in Göttingen and find that embryology is still going strong. As in the 1938 Group there are some eminent biologists here. But the most striking difference between the two photos is the inclusion of many postdocs and graduate students with the promise it brings for the future of the field.

Some curiosa

Together with Elliott Smith, J. T. Wilson and J. P. Hill were part of the Fraternity of the Duckmaloi that pioneered research in monotreme and marsupial embryology. Their heirs are Marilyn Renfree (front row fourth from left) and Karen Lychau Hansen (front row third from right).

Theodore Thomson Flynn also worked on marsupials but later turned to fish. He named one species Gibbonsia erroli after his son, who later achieved fame as the swashbuckling film star Errol Flynn.  

One of the few women in the 1938 Photo is Katherine Jones Hill. The daughter of J. P. Hill, she was an embryologist in her own right. She catalogued the Hill Collection now housed in Berlin. 

Zofia Kielan-Jaworowska

Zofia Kielan-Jaworowska 1925-2015

The current issue of Nature carries an obituary of Professor Zofia Kielan-Jaworowska, the eminent palaeontologist, who died 13 March 2015.

In Pursuit of Mammals

Previously I reviewed the autobiography of this remarkable woman (here) and it remains a fascinating account of her adventurous life and the early history of mammals.