Superfreak of Evolution: The Lizard With a Humanlike Placenta

by Ed Yong
04.09.2012

Trachylepis ivensi nurtures its fetus much like humans do.
Courtesy: Dr. Philipp Wagner

In central Africa, an unassuming little lizard has evolved a spectacular and oddly human feature of gestation: a complex placenta. It is the first time that scientists have observed such an advanced version of this organ connecting the fetus to the womb in nonmammals.

Biologist Alexander Flemming made the anatomical find, announced late last year, while sorting through specimens at the Port Elizabeth natural history museum in South Africa. Flemming and his collaborator, Daniel Blackburn, knew that about 20 percent of lizards give birth to live young, but finding the placenta came as a shock.

Whereas virtually all cold-blooded reptiles supply embryos with nutrients from a large egg yolk, five-inch-long Trachylepis ivensi females ovulate small, yolk-poor eggs that implant in the uterus. As the fetus develops, its tissues become intimately entangled with the blood vessels of its mother, providing ready access to nutrients and oxygen in the mother’s blood. Sound familiar? “The fetal tissues actually invade the uterine ones, much like in humans,” Blackburn says. “It’s totally unexpected.”

Blackburn wants to reconstruct the evolution of this complex organ from simpler versions in other lizards. Doing so might even tell us about how the human placenta evolved. “It goes to show,” he says, “that you never know what diversity may be out there until you look.”

Evolution of Populations

This clip describes the population evolution that we discussed in class and reviews Darwins travels and some evidence for evolution. It also includes the Hardy-Weinberg equation, but while this is good to know it is not something you will be tested on:

http://www.youtube.com/watch?v=MzKQfrb8Dh4

The background music is a little distracting, but the information is good.

Natural Selection

The following video is taken from Cosmos and explores the theory of natural selection.

The following video takes another look at natural selection’s mechanism. Now it is not always perfectly accurate, but it does a pretty good job of outlining the steps of how a trait can be “selected” by nature for passing on.

One more video going over the basics of evolution and natural selection:

One of the key things to keep in mind is our idea of fitness.

Fitness: the ability of an individual to survive and reproduce in its specific environment

Try to relate what is in the videos to the three tenants of evolution by natural selection:

  1. The Struggle for Existence
  2. Survival of the Fittest
  3. Descent With Modification

History of Evolutionary Thought

The following video shows the major steps in evolutionary thought. It goes beyond what we covered in class to give a more complete picture, but to my estimation it has one flaw… it is missing Malthus.

The following is a clip on the work of Thomas Malthus.

The following clip is on The Impact of Evolutionary Thought on the Social Sciences. It is about 48 minutes, but interesting to think about and how this relates to what we had discussed in bioethics:

Amber trapped dinosaur feathers at different stages in their evolution

We now have enough fossils of feathered dinosaurs to fill entire museums. These specimens have beautifully recorded the history of feather evolution but Ryan McKellar from the University of Alberta has found another narrator for that tale: amber.

Amber is actually fossilised tree resin, and some of it contains feathers dropped by dinosaurs. McKellar, working with Phillip Currie, studied beautiful cache of 11 such pieces that had been recovered from Grassy Lake, Canada many years earlier. “These were chance finds when we were preparing the amber to look for insects,” says Currie. “About half a dozen specimens were in my “research to do” specimen cabinet when Ryan ran across some more in his work.”

These 70-million-year old specimens hail from the late Cretaceous period and they are far more diverse than other amber-trapped feathers from the same period. It’s impossible to say who the feathers came from but given that some of them are far simpler than anything seen in known birds, McKellar suspects that they came from other types of dinosaur instead. After all, such species are far more common in Alberta’s fossil beds than true birds are.

Together, the amber feathers encompass the entire evolutionary history of feathers in four different stages, from simple filaments to flight-capable plumes. “It fleshes out a framework that was already evident from the feathered dinosaurs, although the preservation is far more spectacular in terms of detailed preservation and even colours,” says Currie. The slideshow below shows the various types.

Reference: McKellar, Chattertton, Wolfe & Currie. 2011. A Diverse Assemblage of Late Cretaceous Dinosaur and Bird Feathers from Canadian Amber.

http://dx.doi.org/10.1126/science.1203344

Fossil find puts ‘Lucy’ story on firm footing

By Jonathan Amos
Science correspondent, BBC News

New fossil evidence seems to confirm that a key ancestor of ours could walk upright consistently – one of the major advances in human evolution.

The evidence comes in the form of a 3.2 million-year-old bone that was found at Hadar, Ethiopia.

Its shape indicates the diminutive, human-like species Australopithecus afarensis had arches in its feet.

Arched feet, the discovery team tells the journal Science, are critical for walking the way modern humans do.

“[The bone] gives a glimpse of foot anatomy and function,” explained William Kimbel, director of the Institute of Human Origins at Arizona State University, US.

“It is the fourth metatarsal bone, which resides on the outside of the middle part of your foot, and which helps support the well-developed arches of the foot that we see in the soles of modern human feet.

“The bone that was recovered from the Hadar site has all the hallmarks of the form and function of the modern human foot,” he told the BBC.

Arch types
Palaeo-scientists knew A. afarensis spent some of its time standing tall; that much has been clear since 1974 when they first examined a skeleton of the species, famously dubbed “Lucy”, also found near the village of Hadar in the Ethiopian rift valley.

But the absence of important foot bones in all of the specimens uncovered to date has made it difficult for researchers to understand precisely how much time Lucy and her kin spent on their feet, as opposed to moving through the branches of trees.

Human feet are very different from those of other primates. They have two arches, longitudinal and transverse.

These arches comprise the mid-foot bones, and are supported by muscles in the soles of the feet.

This construction enables the feet to perform two critical functions in walking. One is to act as a rigid lever that can propel the body forwards; the other is to act as a shock absorber as the feet touch the ground at the end of a stride.

In our modern ape cousins, the feet are more flexible, and sport highly mobile large toes that are important for gripping branches as the animals traverse the tree tops.

Professor Kimbel and colleagues tell Science journal that the feet of A. afarensis’ say a lot about the way it lived.

It would have been able to move across the landscape much more easily and much more quickly, potentially opening up broader and more abundant supplies of food, they say.

“Lucy’s spine has the double curve that our own spine does,” Professor Kimbel said.

“Her hips functioned much as human hips do in providing balance to the body with each step, which in a biped of course means that you’re actually standing on only one leg at a time during striding.

“The knees likewise in Lucy’s species are drawn underneath the body such that the thighbone, or femur, angles inwards to the knees from the hip-joints – as in humans.

“And now we can say that the foot, too, joins these other anatomical regions in pointing towards a fundamentally human-like form of locomotion in this ancient human ancestor.”

A. afarensis is thought to have existed between about 2.9 million and 3.7 million years ago, and the Hadar area has yielded hundreds of fossil specimens from the species.

Long road
Commenting on the latest research, Professor Chris Stringer, a palaeoanthropologist at London’s Natural History Museum, said scientists were gradually filling in the detail of this creature’s position in the human origins story.

“Bipedalism in Lucy is established, but there has been an issue about how much like our own that bipedalism was,” he told BBC News.

“Was it a more waddling gait or something more developed?

“And certainly there’s evidence in the upper body that the Australopithecines still seemed to have climbing adaptations – so, the hand bones are still quite strongly curved and their arms suggest they’re still spending time in the trees.

“If you are on the ground all the time, you need to find shelter at night and you are in a position to move out into open countryside, which has implications for new resources – scavenging and meat-eating, for example.

“If the Australopithecines were on that road, they were only at the very, very beginning of it.”

Fossil female pterosaur found with preserved egg

By Jonathan Amos
Science correspondent, BBC News
It was the males which carried a crest, the latest research published in Science magazine suggests

For fossil hunters, it represents one of those breakthrough moments.

A pterosaur has been found in China beautifully preserved with an egg.

The egg indicates this ancient flying reptile was a female, and that realisation has allowed researchers to sex these creatures for the first time.

Writing in Science magazine, the palaeontologists make some broad statements about gender differences in pterosaurs, including the observation that only males sported a head-crest.

David Unwin, a palaeobiologist in the Department of Museum Studies at the University of Leicester, was part of the research team.

He told the BBC the discovery was astonishing: “If somebody had said to me a few years back that we would find this kind of association, I would just have laughed and said, ‘yeah, maybe in a million years’, because these sorts of things are incredibly rare.”

Pterosaurs, also sometimes referred to as pterodactyls, dominated the skies in the Mesozoic Era, 220-65 million years ago. Although reptiles like the dinosaurs were plodding on the ground below them, they were not actually dinosaurs themselves – a common misconception.

This particular specimen has been dated to about 160 million years ago.

It was found by Junchang Lü and colleagues and excavated from sedimentary rocks in the famous fossil-hunting grounds of Liaoning Province in China. Liaoning has yielded many of the great finds in recent years, including a series of feathered dinos that have transformed thinking on bird evolution.

The new creature is from the Darwinopterus genus, or grouping, but has been dubbed simply as “Mrs T” (a contraction of “Mrs Pterodactyl”) by the research team.

The state of the egg’s shell suggests it was well developed and that Mrs T must have been very close to laying it when she died.

She appears to have had some sort of accident as her left forearm is broken. The researchers speculate she may have fallen from the sky during a storm or perhaps a volcanic eruption, sunk to the bottom of a lake and then been preserved in the sediments.

“The most important thing about this particular individual is that she has a relatively large pelvis compared to other individuals of the same pterosaur, Darwinopterus,” explained Dr Unwin.

“This seems quite reasonable – females lay eggs, they probably need a slightly wider pelvis. And then the really exciting thing is that she has a skull which lacks any kind of adornment or decoration whatsoever. When we look at other individuals of Darwinopterus, we find quite a few individuals with a large crest on the skull.

“We’re very confident now that we’re dealing with two genders here – males with big crests and small hips, and females with no crest on the skull and large hips.”

The female fossil partially prepared (A). After being fully prepared (B), the egg is clear to see (red circle)

Jonathan.Amos-INTERNET@bbc.co.uk

Audio included on original page: http://www.bbc.co.uk/news/science-environment-12242596