Two new studies undermine “over-simplistic models of human evolution”

by Mar 18, 2016 1:03am GMT

The relationship between early human groups was incredibly complicated.

You’ve probably heard the story about how Neanderthals were living in Europe for hundreds of thousands of years, when suddenly a bunch of Homo sapiens came pouring out of Africa about 70 thousand years ago. Thirty thousand years later, pretty much all the Neanderthals were dead. Many anthropologists believe that Homo sapiens killed off our large-browed cousins in a quest to dominate the Eurasian continent. But over the past 10 years, that view has changed radically thanks to new techniques for sequencing ancient DNA.

Now, two new studies make it even less likely that modern humans killed off the Neanderthals. Instead, we interbred with them at least three separate times, and our ancestors were likely sharing tools with them half a million years ago.

A mysterious common ancestor

Writing in Nature, a team of scientists recount how they carefully sequenced the DNA from a mysterious group of 430-thousand-year-old humans found in Sima de los Huesos, a cavern in Spain’s Atapuerca mountains. Thanks to careful preservation of the remains, they were able to extract both mitochondrial and nuclear DNA, allowing them to analyze genetic contributions from the group’s mothers and fathers. What they discovered has upended the classic story of how Neanderthals got to Europe, and when. The Sima hominins are clearly early Neanderthals, living in Spain far earlier than expected.

Paleolithic sexytimes reveal that Homo sapiens made it out of Africa earlier than we thought.
Previously, scientists believed the last common ancestor of Neanderthals and modern humans was Homo heidelbergensis, an early human who lived about 700 to 200 thousand years ago. But the Sima hominins reveal that humans and Neanderthals must have diverged between 550 thousand and 765 thousand years ago, a timeframe that eliminates H. heidelbergensis as a possible progenitor. Paleoanthropologist Maria Martinón-Torres told Nature News that the common ancestor of Neanderthals and modern humans probably lived about 900 to 700 thousand years ago. One possibility is Homo antecessor, whose 900-thousand-year-old remains have been found in Spain.
Making things more confusing, the Sima humans have mitochondrial DNA that appears to be from Denisovans, another group of early humans that settled in Europe hundreds of thousands of years before modern humans did. Writing in Nature News, Ewan Callaway explains that Matthias Meyer, an author on the new study, “now favors the hypothesis that an as yet unknown species from Africa migrated to Eurasia and bred with Neanderthals, replacing the mitochondrial DNA lineages. (Supporting this idea, stone-tool technologies spread from Africa to Eurasia around half a million years ago, and again 250,000 years ago).” In other words, these early humans leaving Africa weren’t killing Neanderthals; they were sharing tools and families with them.

More Neanderthal sex than ever

Even tiny groups of humans would have had the tech to out-compete Neanderthals.
Meyer’s findings fit nicely with other recent studies, which show that even when modern humans left Africa, they weren’t killing their distant human cousins. Instead, they had children with Neanderthals and Denisovans, in several distinct events. If the meeting between humans and Neanderthals had been genocidal, it’s unlikely we would see patterns that show Neanderthal and Denisovan DNA enter the modern human genome multiple times. A paper that has just come out in Science explains how modern human DNA contains clear evidence that our ancestors got busy with Neanderthals and Denisovans at least three different times in the past 100 thousand years.
The researchers analyzed DNA taken from 1,523 people spread across the globe, looking for traces of Neanderthal and Denisovan DNA sequences. What they found was that different populations of people had distinct, different collections of DNA from Neanderthals. They write:

Collectively, these data suggest Neanderthal admixture occurred at least three distinct times in modern human history. Although most South Asian populations show shared histories of archaic admixture, we find significant evidence of differential Neanderthal admixture between some European and East Asian populations.
In other words, modern humans didn’t sweep out of Africa, killing everything in their paths. They settled down with the locals, many different times. Evolutionary biologist Carles Laleuza-Fox, who was not involved in the study, told The New York Times’ Carl Zimmer, “This is yet another genetic nail in the coffin of our over-simplistic models of human evolution.” These papers also testify to how long different groups of humans have been intermingling, sharing ideas and hearths. Even though humans are notorious for hating and killing strangers, there’s no denying that migration is written into our DNA, as well as a history of embracing people who are different.

Nature, 2016. DOI: 10.1038/nature17405

Science, 2016. DOI: 10.1126/science.aad9416

Continue reading Two new studies undermine “over-simplistic models of human evolution”

Fossil wood

Occasionally some rarer objects turn up on fOssilfinder. One such object is that of a piece of fossil wood.

Fossil wood
The paler piece in the bottom of this image is the piece of fossil wood. It stands out amongst the many other sandstone pieces in this image.

Fossil wood has a streaked appearance to it and if broken in cross section tree rings can be seen in the fossil too. There are a variety of different woods, including some from palm trees, although it would be difficult to identify the type of wood from the resolution in the fossil finder images.

Towards the eastern margin there are fossil sites in which fossil wood is abundant. Pieces of wood turn up in the fossil exposures that have been carried in streams or sometimes carried in as raw material for stone tools.

New species of Homo discovered in South Africa

Today the discovery of a new species in our primate lineage has been announced. Homo naledi is the new name for the species, marking its assignment as a close ancestral species. This find is based on the discovery of more than 1500 specimens from Rising Star Cave near Johannesburg in South Africa.

Two articles have been published on the finds. The first describing the new species and the second describing the context of its discovery.

Stomatolites

Stromatolites are trace fossils of the biofilms of blue-green algae that grow around an object such as a shell or oyster. As the algal filaments grow outwards, sediment is continually deposited and accreted onto the surface of the algal mat. The stromatolite increases in size and forms distinctive concentric rings if you break the stromatolite in cross section.

This is an earlier stage of stromatolite development that is growing around half of an inch long bivalve.
This is an earlier stage of stromatolite development that is growing around half of an inch long bivalve.

The outer surface of the fossil stromatolites look quite knobbled in their appearance. These round boulders can sometimes be quite large in the fossil record at Lake Turkana, sometimes reaching a meter wide.

Some of these stromatolites can be up to a meter in diameter.
Some of these stromatolites can be up to a meter in diameter.

Stromatolites grow in shallow, hypersaline pools in shore settings, inhospitable to molluscs and other invertebrates that might otherwise graze on the algae. There are no modern examples of stromatolites found today at Lake Turkana, however there are modern marine stromatolites that are in lagoons in Shark Bay in Australia. A 3D model of a stromatolite from 0.7 million year deposits at Lake Turkana can be seen here;

This is a cross section of a stromatolite that has grown up around an oyster shell.
This is a cross section of a stromatolite that has grown up around an oyster shell.

 

Finding fossils the traditional way

 

Finding fossils and other geological features is best done with a trained eye and on foot, where you can bend down and look more closely at your discovery and possibly pick it up and look at it from a different angle. For many year of fieldwork at Lake Turkana, trained fossil hunters have camped in the fossil areas and go out on daily prospection missions, walking over the landscapes looking for fragments that they identify and document on the surface. 

The team walk across the fossil exposures looking for fossil fragments on the surface.
The team walk across the fossil exposures looking for fossil fragments on the surface.
Fossil hunter bending down to inspect a piece on the surface. Photo Credit: Mike Hettwer
Fossil hunter bending down to inspect a piece on the surface. Photo Credit: Mike Hettwer

The team walk over the fossil exposures looking for fossil fragments and geological indicators, similar to the way that you can search for identifiable objects in the images on fOssilfinder. The fossil hunters have the added advantage of being able to pick up a specimen and turn it over to get a better look at it which can be most helpful in its identification. If a specimen is collected then it is essential that its exact position is recorded. Fossils should not be moved out of context.

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These coloured tracks on the Google image, mark the ground covered by the team over one week. These tracks are a useful indication of which areas need to be further covered.

When a discovery is made by a member of the team, a GPS position is taken as well as a digital photograph and some basic notes about the find. It is also given a unique field number, which is written on the fossil and on a small stone beside this so that the number is visible in the photograph for the database. You find many fossil fragments and artifacts on the surface, some of which are taken out of context as they are found in a dry stream bed or have been eroded out of sediments from above that have since washed away. It could therefore be a younger age than that of the deposits that it is sitting on. Ideally we want to discover a fossil that is in its original context and excavate this carefully to recover it.

This is an image of the excavation of a large and complete elephant skull.
This is an image of the excavation of a large and complete elephant skull.

This discovery of an elephant skull was made by spotting the teeth that were visible on the surface but after excavation the enormous skull was exposed. This was eventually jacketed in Plaster of Paris and lifted safely into the back of a car. You can see a video of this here

Once an important discovery is made, it is collected by the team, following strict procedures. Detailed notes are made about the surrounding sediment and rocks. We are interested to know if there are any nearby sandstones, what texture these sandstones are (coarse grained, fine grained, laminated, bioturbated etc.). We also document the presence of root-casts, rhizoliths, calcrete and quartz pebbles. These are good indicators of palaeoenvironment. We are interested in the presence of snails, fragments of fossil turtle, fish or crocodile, and also the type and colour of the substrate (if it is a silt or a clay, sand or a volcanic ash). These are some of the characteristics that we describe during collection of a specimen. We are hoping that you we can replicate this skill by training new eyes in finding fossils and other interesting features in these images.

Is there more to the genus Homo than we think?

Almost 300 years ago, Linnaeus defined our genus Homo (and its species Homo sapiens) with the noncommittal words nosce te ipsum (know thyself). Since then, fossil and molecular biology studies have provided insights into its evolution, yet the boundaries of both the species and the genus remain as fuzzy as ever, new fossils having been rather haphazardly assigned to species of Homo, with minimal attention to details of morphology.

A paper published in today’s issue of science magazine by Jeffrey Schwartz and Ian Tattersall argues that, historically, fossil specimens have been “shoehorned” into Homo species “without regard to their physical appearance”. They overview recent examples of debate to illustrate significant differences in the ways that researchers are willing to group specimens. They also picture several fossil specimen examples of homo to highlight marked differences between them. The paper calls for a revisit of criteria for species recognition. The reason being that historical classification schemes, and subjective assignment of new fossils, may be limiting how we can build testable models of human evolution.

The Science perspectives paper can be found here: “Defining the genus Homo.” Science 28 August 2015: Vol. 349 no. 6251 pp. 931-932 DOI: 10.1126/science.aac6182

1984: The discovery of Nariokotome Boy

This skull and mandible belong to the 1.6 million year old skeleton of Homo erectus found at Nariokotome on the west side of Lake Turkana, sometimes referred to as the “Turkana Boy” or the “Nariokotome Boy”. This discovery was made by Kamoya Kimeu, on a Sunday morning in August 1984. He was walking up a gentle slope on the southern side of the seasonal Nariokotome River when he spotted a matchbox sized skull fragment, which he recognized as belonging to a human ancestor. No one could have imagined that such a complete specimen would be recovered. As the excavation of the hillside progressed more pieces of skull, teeth, and then ribs began to be uncovered. This turned into one of the most exciting excavations of arguably the most important fossil discovered in east Africa.
Due to its completeness, this skeleton provides unprecedented insight into the body shape, brain size and development of Homo erectus. The Turkana Boy was surprisingly tall, 5’3” (1.6 meters) although he was still an adolescent. He had a slender body well adapted to living in hot climates. Homo erectus was the first human ancestor to migrate out of Africa 1.8 million years ago. Several specimens have also been recovered from sites in China, Indonesia, and Dmanisi in the Republic of Georgia.

A 3D scan of this fossil and others can be found at africanfossils.org