Pieces of Homo naledi story continue to puzzle

Another twist in the Homo naledi tale

Age, place on hominid family tree, how fossils ended up in hard-to-reach cave remain unknown

DEEP EVOLUTION  Homo naledi fossils were found in South Africa’s Dinaledi Chamber, outlined here. Researchers debate whether this species dropped its dead through a shaft into the underground space, creating the array of bones shown on the chamber floor.
P.H.G.M. DIRKS ET AL/ELIFE 2015 (CC BY 4.0

Homo naledi, a rock star among fossil species in the human genus, has made an encore. Its return highlighted debate over whether this hominid was a distinct Homo species that purposefully disposed of at least some of its dead.

H. naledi made worldwide headlines last year when researchers announced the discovery of an unusually large collection of odd-looking Homo fossils in the bowels of a South African cave system. Presentations at the annual meeting of the American Association of Physical Anthropologists on April 16 underscored key uncertainties about the hominid.

One of the biggest mysteries: H. naledi’s age. Efforts are under way to date the fossils and sediment from which they were excavated with a variety of techniques, said paleoanthropologist John Hawks of the University of Wisconsin–Madison. An initial age estimate may come later this year if different dating techniques converge on a consistent figure. A solid date for the fossils is essential for deciphering their place in Homo evolution and how the bones came to rest in a nearly inaccessible cave.

Some presenters reasserted that H. naledi intentionally dropped dead comrades into an underground chamber, where their bones were later found by cave explorers and then scientists. But others raised questions. Even paleoanthropologist and team leader Lee Berger of the University of the Witwatersrand in Johannesburg hedged his bets.

“It’s way too early to tell how H. naledi bodies got in the chamber,” Berger said.

Berger’s group recovered 1,550 H. naledi fossils from a minimum of 15 individuals of all age groups (SN: 10/3/15, p. 6). Slender researchers wended through narrow passageways in South Africa’s Rising Star cave system and squeezed down a vertical chute to reach pitch-dark Dinaledi Chamber. There, they found hominid fossils scattered on the floor and in a shallow, 20-centimeter-deep excavation.

Berger’s team assigned the bones to H. naledi based on an unexpected mix of humanlike features and traits typical of Australopithecus species from more than 3 million years ago.

Fossil analyses presented at the meeting challenged a suggestion by some researchers, both before and during the meeting, that H. naledi actually represents a variant of Homo erectus, a species known to have existed by 1.8 million years ago (SN: 11/16/13, p. 6).

H. naledi possessed a shoulder unlike those of other Homo species, said team member Elen Feuerriegel of the Australian National University in Canberra. The Rising Star hominid’s collarbone and upper arm bone resemble corresponding Australopithecus bones, she reported. H. naledi’s shoulder blades must have been positioned low and behind the chest, an arrangement more conducive to climbing trees than running long distances.

H. naledi’s hand was built both for climbing and gripping stone implements, said Tracy Kivell of the University of Kent in England. Her analysis of 150 hand bones, including a nearly complete hand, showed a humanlike wrist and thumb combined with Australopithecus-like curved fingers.

MIXED GRIP H. naledi’s hand combines a humanlike wrist and thumb with curved fingers characteristic of tree-climbing hominids. This unusual hand design sets H. naledi apart from other Homo species, researchers argue.
L.R. BERGER ET AL/ELIFE 2015 (CC BY 4.0)

H. naledi’s curved toes and flaring pelvis also recall Australopithecus. Still, a preliminary lower-body reconstruction — incorporating fossil evidence of humanlike legs, knees and feet — suggests H. naledi walked almost as well as modern humans do, said Zach Throckmorton of Lincoln Memorial University in Harrogate, Tenn.

West Asian H. erectus and H. naledi share several tooth features as well as relatively small braincases. In addition, adult H. naledi stood an estimated 147 centimeters tall (4 feet, 10 inches), within the height range for West Asian H. erectus. “That complicates matters,” said Christopher Walker of Duke University. Upper-body features that Berger’s team considers characteristic of H. naledi, such as the upper arm’s shape, possibly occurred in West Asian H. erectus as well, added Witwatersrand’s Tea Jashashvili, who has studied those finds.

Explaining how H. naledi bones ended up in Dinaledi Chamber is also complicated. Ongoing studies of sediment and rock indicate that there was never a direct opening to the underground fossil site from above, said Witwatersrand’s Marina Elliott.

Bones from some body parts, including five feet, three hands and part of a backbone, were found aligned as they would have been in living individuals, indicating at least some bodies reached the chamber intact, Hawks said. Curiously, some sets of aligned bones were found beneath scattered bones from diverse individuals.

If the dead were dropped down a vertical chute into Dinaledi Chamber, bodies on top would have been least damaged and most likely to retain aligned bones. Along with that mystery, some sets of aligned bones somehow ended up far from the chute’s opening, Berger said.

An alternative entrance to Dinaledi Chamber possibly existed in the past, Witwatersrand’s Aurore Val  asserted online March 31 in the Journal of Human Evolution. Beetles or snails that damaged some H. naledi bones don’t inhabit dark, underground caves, Val argues. Such damage probably occurred on the surface or in a nearby, once-accessible part of the cave system, she proposes.

The surfaces of many H. naledi fossils had been worn down enough to have possibly erased predators’ tooth marks and signs of animal trampling, which would be additional signs that another entrance to the chamber once existed, Val says.

Given the large number of isolated and broken H. naledi fossils, bodies or body parts may have entered the chamber long after death, in Val’s view. Perhaps water from another part of the cave system carried bodies into Dinaledi Chamber, she speculates.

Geologic studies show that water occasionally reached the chamber and mildly eroded sediment, Berger said. But he doubts water washed bones into Dinaledi Chamber. “Even if there was another entrance to the chamber, it still allowed access only to Homo naledi,” Berger argued. No remains of any other animals have been found in the cave.

Like any rock star of lasting impact, the South African hominid plans to wow fans with new material. “Thousands of Homo naledi fossils are almost certainly left in the underground chamber,” Berger said.

Editor’s note: This story was updated April 25, 2016, to correct the identification of organisms that have damaged some H. naledi bones.


Tomado de: https://www.sciencenews.org/article/pieces-homo-naledi-story-continue-puzzle

Neandertales, denisovanos y sapiens: sexo y adaptación local

Nuevas evidencias de que los antiguos cruces entre las tres especies tuvieron consecuencias evolutivas.

Localización geográfica de las 159 poblaciones estudiadas. SCIENCE

Estamos tan acostumbrados a ser los únicos humanos sobre la Tierra que casi no podemos imaginar un pasado en que, viajando desde África hacia un mundo desconocido, lo más fácil era encontrar por ahí a otros de los nuestros, otras especies del género Homoque compartían con nosotros un pasado olvidado, y con las que, según sabemos ahora, no nos importaba compartir el sueño de una noche de verano. Sin considerarlo animalismo, y sin que nuestra lógica más profunda, la genética, lo viera inconveniente tampoco, puesto que de aquellos polvos han venido estos lodos que la ciencia revela ahora en nuestro genoma.

Según la última investigación de 1.523 genomas de personas de todo el mundo, incluidos por primera vez los de 35 melanesios, los neandertales se cruzaron no una, sino tres veces (en tres épocas distintas), con diversas poblaciones de humanos modernos. Solo se libraron los africanos, por la sencilla razón de que los neandertales no estaban allí. Los melanesios actuales llevan ADN de otra especie arcaica, los misteriosos denisovanos que vivían en Siberia hace 50.000 años, pero ni por esas se libraron de la promiscuidad neandertal: sus genomas actuales llevan las marcas inconfundibles tanto de neandertales como de denisovanos.

Y un premio de consolación: los genes de la evolución del córtex, la sede de la mente humana, son enteramente nuestros, de los Homo sapiens. Lo demás parecen ser adaptaciones al clima local. Son los resultados que 17 científicos de la Universidad de Washington en Seattle, la Universidad de Ferrara, el Instituto Max Planck de Antropología Evolutiva en Leipzig y el Instituto de Investigación Médica de Goroka, en Papúa Nueva Guinea, entre otros, han presentado en Science.

Los genomas se suelen medir en megabases, o millones de bases (las letrasdel ADN, gatacca…). El genoma humano tiene 3.235 megabases. De ellas, 51 megabases son arcaicas en los europeos, 55 en los surasiáticos y 65 en los asiáticos orientales. Casi todas esas secuencias arcaicas son de origen neandertal en estas poblaciones. En contraste, los melanesios presentan un promedio de 104 megabases arcaicas, de las que 49 son neandertales, y 43 son denisovanas (las 12 restantes son ambiguas de momento). Son solo números, aunque dan una idea del grado de precisión que ha alcanzado la genómica humana.

Pero el diablo mora en los detalles. Las secuencias arcaicas no están distribuidas de manera homogénea por el genoma, ni mucho menos. Hay zonas donde están muy poco representadas, es decir, donde hay tramos de 8 megabases o más sin una sola letra neandertal o denisovana. Estos tramos de puro ADN moderno, o sapiens, son ricas en genes implicados en el desarrollo del córtex cerebral –la sede de la mente humana— y el cuerpo estriado (o núcleo estriado), una región interior del cerebro responsable de los mecanismos de recompensa, y por tanto implicada a fondo en planear acciones y tomar decisiones

Que los genes implicados en estas altas funciones mentales estén limpios de secuencias neandertales o denisovanas no puede ser casual, según los análisis estadísticos de los autores. El hecho implica, probablemente, que la presencia de ADN arcaico allí ha resultado desventajosa durante los últimos 50 milenios, y por tanto ha resultado barrida por la selección natural.

Entre los genes modernos se encuentra el famoso gen del lenguaje, FOXP2, lo que vuelve a plantear dudas sobre la capacidad de lenguaje de los neandertales. Que la secuencia de este gen sea idéntica en neandertales y sapiens se ha considerado una evidencia de que los neandertales hablaban, pero los genes son más que su secuencia de código (la que se traduce a proteínas): hay además zonas reguladoras esenciales, las que le dicen al gen dónde, cuándo y cuánto activarse. Otros genes puramente modernos son los implicados, cuando mutan, en el autismo.

También son interesantes las regiones genómicas contrarias, es decir, las que están particularmente enriquecidas en genes neandertales o denisovanos. Los genomas melanesios han revelado 21 regiones de este tipo que muestran evidencias de haber sido favorecidas por la selección natural. Muchas de ellas contienen genes implicados en el metabolismo (la cocina de la célula), como el de la hormona GCG, que incrementa los niveles de glucosa en sangre, o el de la proteína PLPP1, encargada de procesar las grasas; también hay cinco genes implicados en la respuesta inmune innata, la primera línea de defensa contra las infecciones.

Todo ello refuerza los indicios anteriores de que los cruces de nuestros ancestros sapiens con las especies arcaicas que encontraron durante sus migraciones fuera de África tuvieron importancia para adaptarse a las condiciones locales: clima, dieta e infecciones frecuentes en la zona. Tiene sentido, desde luego.

Tomado de: http://elpais.com/elpais/2016/03/25/ciencia/1458906491_911456.html

Proof of evolution that you can find on your body

Food processing

A recreation of how early humans managed to eat a diet of meat hundreds of thousands of years before they had fire to cook it with, shows an ingenious use of tools to cut down on chewing time.

You are what you eat. Not only that, but you are what your ancestors ate, when they ate it, and what they did to it first. One of the many peculiarities that set humans apart from other animals is that eating is more than just stuffing something into our mouths.

True, the human diet is astonishingly eclectic, but this wide range is tempered by elaborate preparation. No other animal, for example, exposes prospective food items to prolonged heating, a habit we call ‘cooking’. It’s now generally thought that cooking was central to the evolution of modern humans, prompting a massive reduction in tooth size and chewing muscles, alongside a marked increase in available nutrients, more time to spend doing other things besides chewing, and even an expansion of the brain.

There is — as always — a catch. Cooking requires fire, and there is scant evidence for the regular use of fire before around 500,000 years ago. Homo erectus, the first hominin to even begin to approach modern humans in stature, brain size and masticatory apparatus, appeared around 1.5 million years earlier than that. Homo erectus was a regular carnivore, a habit that has stayed with us and is believed to be necessary to our modern diet (see Nature 531, S12–S13; 2016).

How did H. erectus manage to consume meat without cooking it? As Katherine Zink and Daniel Lieberman explore in a paper online in Nature (see http://dx.doi.org/10.1038/nature16990), raw meat is tough and practically impossible to break down into swallowable pieces just by chewing it. Side orders of roots and tubers can be crunched, but only if you are prepared to put in the hours. A lot of hours. About 40,000 chews a day, which, at a ruminative rate of 1 chew per second, adds up to 11 hours. That’s almost a whole day gone, just chewing. That’s no issue for many baseball players or football managers, perhaps, but H. erectus had better things to do.


The new study squares the circle by showing that tools equivalent to knives, mortars and pestles entered the kitchen a long time before the oven. Stone tools date back to at least 3.3 million years ago (S. Harmand et al. Nature 521, 310–315; 2015). A freshly struck flake of stone makes short work of slicing raw meat into morsels, and a lump of rock can be used to pound roots and tubers into a paste.

Work with people today has put numbers on these gains. When meat is sliced and roots are pounded, a prehistoric diet of 2,000 kilocalories per day (one-third raw goat and two-thirds raw yams, carrots and beets) can be achieved with 2.5 million fewer chews a year than if the items are unprocessed. That’s an entire month spent not chewing — presumably enough to explain the reduction in tooth size and masticatory muscle mass of H. erectus compared with earlier, more masticatory species, as well as the increase in brain size allowed by the release of more nutrients. And what does one do with one’s mouth when not chewing? One talks a lot, of course. Preferably to other people.

Our ancestors probably also ate fruits and berries, fish and shellfish, nuts, bone marrow, liver and brains, all of which are highly nutritious. But some of those foods need a deal of slicing and pounding to get at. Nuts have hard shells, as do shellfish, by definition; marrow and brains require (there is no delicate way to put this) the smashing of bones and skulls. Many animals are known to use simple tools to acquire food of that sort. But the release of nutrients from muscle by an animal with teeth more suitable for crushing than slicing required the application of some early food technology.


Cooking, when it came, enabled yet more efficient nutrient release, and provided other benefits such as the killing of any harmful parasites that raw meat might contain, as well as the gathering of sociable people round a hearth to swap gossip, watch celebrity chefs on TV and share pictures of their cats on the Internet, if only as a way of using up all that time not spent chewing the fat. But cooking did not start this. It merely accelerated a culinary tradition already millions of years old.

Nature 531, 139 (10 March 2016) doi:10.1038/531139a

Tomado de: http://www.nature.com/news/food-processing-1.19513