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Tuesday, October 6, 2015

Stefan–Boltzmann law


From Wikipedia, the free encyclopedia
 

Graph of a function of total emitted energy of a black body j^{\star} proportional to its thermodynamic temperature T\,. In blue is a total energy according to the Wien approximation,

j^{\star}_{W} = j^{\star} / \zeta(4) \approx 0.924 \, \sigma T^{4} \!\,

The Stefan–Boltzmann law, also known as Stefan's law, describes the power radiated from a black body in terms of its temperature. Specifically, the Stefan–Boltzmann law states that the total energy radiated per unit surface area of a black body across all wavelengths per unit time (also known as the black-body radiant exitance or emissive power), j^{\star}, is directly proportional to the fourth power of the black body's thermodynamic temperature T:
j^{\star} = \sigma T^{4}.
The constant of proportionality σ, called the Stefan–Boltzmann constant or Stefan's constant, derives from other known constants of nature. The value of the constant is
\sigma=\frac{2\pi^5 k^4}{15c^2h^3}= 5.670373 \times 10^{-8}\, \mathrm{W\, m^{-2}K^{-4}},
where k is the Boltzmann constant, h is Planck's constant, and c is the speed of light in a vacuum. Thus at 100 K the energy flux is 5.67 W/m2, at 1000 K 56,700 W/m2, etc. The radiance (watts per square metre per steradian) is given by
L = \frac{j^{\star}}\pi = \frac\sigma\pi T^{4}.
A body that does not absorb all incident radiation (sometimes known as a grey body) emits less total energy than a black body and is characterized by an emissivity, \varepsilon < 1:
j^{\star} = \varepsilon\sigma T^{4}.
The irradiance j^{\star} has dimensions of energy flux (energy per time per area), and the SI units of measure are joules per second per square metre, or equivalently, watts per square metre. The SI unit for absolute temperature T is the kelvin. \varepsilon is the emissivity of the grey body; if it is a perfect blackbody, \varepsilon=1. In the still more general (and realistic) case, the emissivity depends on the wavelength, \varepsilon=\varepsilon(\lambda).

To find the total power radiated from an object, multiply by its surface area, A:
P= A j^{\star} = A \varepsilon\sigma T^{4}.
Wavelength- and subwavelength-scale particles,[1] metamaterials,[2] and other nanostructures are not subject to ray-optical limits and may be designed to exceed the Stefan–Boltzmann law.

History

The law was deduced by Jožef Stefan (1835–1893) in 1879 on the basis of experimental measurements made by John Tyndall and was derived from theoretical considerations, using thermodynamics, by Ludwig Boltzmann (1844–1906) in 1884. Boltzmann considered a certain ideal heat engine with light as a working matter instead of gas. The law is highly accurate only for ideal black objects, the perfect radiators, called black bodies; it works as a good approximation for most "grey" bodies. Stefan published this law in the article Über die Beziehung zwischen der Wärmestrahlung und der Temperatur (On the relationship between thermal radiation and temperature) in the Bulletins from the sessions of the Vienna Academy of Sciences.

Examples

Temperature of the Sun

With his law Stefan also determined the temperature of the Sun's surface. He learned from the data of Charles Soret (1854–1904) that the energy flux density from the Sun is 29 times greater than the energy flux density of a certain warmed metal lamella (a thin plate). A round lamella was placed at such a distance from the measuring device that it would be seen at the same angle as the Sun. Soret estimated the temperature of the lamella to be approximately 1900 °C to 2000 °C. Stefan surmised that ⅓ of the energy flux from the Sun is absorbed by the Earth's atmosphere, so he took for the correct Sun's energy flux a value 3/2 times greater than Soret's value, namely 29 × 3/2 = 43.5.

Precise measurements of atmospheric absorption were not made until 1888 and 1904. The temperature Stefan obtained was a median value of previous ones, 1950 °C and the absolute thermodynamic one 2200 K. As 2.574 = 43.5, it follows from the law that the temperature of the Sun is 2.57 times greater than the temperature of the lamella, so Stefan got a value of 5430 °C or 5700 K (the modern value is 5778 K[3]). This was the first sensible value for the temperature of the Sun. Before this, values ranging from as low as 1800 °C to as high as 13,000,000 °C were claimed. The lower value of 1800 °C was determined by Claude Servais Mathias Pouillet (1790–1868) in 1838 using the Dulong-Petit law. Pouillet also took just half the value of the Sun's correct energy flux.

Temperature of stars

The temperature of stars other than the Sun can be approximated using a similar means by treating the emitted energy as a black body radiation.[4] So:
L = 4 \pi R^2 \sigma {T_e}^4
where L is the luminosity, σ is the Stefan–Boltzmann constant, R is the stellar radius and T is the effective temperature. This same formula can be used to compute the approximate radius of a main sequence star relative to the sun:
\frac{R}{R_\odot} \approx \left ( \frac{T_\odot}{T} \right )^{2} \cdot \sqrt{\frac{L}{L_\odot}}
where R_\odot, is the solar radius, and so forth.

With the Stefan–Boltzmann law, astronomers can easily infer the radii of stars. The law is also met in the thermodynamics of black holes in so-called Hawking radiation.

Temperature of the Earth

Similarly we can calculate the effective temperature of the Earth TE by equating the energy received from the Sun and the energy radiated by the Earth, under the black-body approximation. The amount of power, ES, emitted by the Sun is given by:
E_S = 4\pi r_S^2 \sigma T_S^4
At Earth, this energy is passing through a sphere with a radius of a0, the distance between the Earth and the Sun, and the energy passing through each square metre of the sphere is given by
E_{a_0} = \frac{E_S}{4\pi {a_0}^2}
The Earth has a radius of rE, and therefore has a cross-section of \pi r_E^2. The amount of solar power absorbed by the Earth is thus given by:
E_{abs} = \pi r_E^2 \times E_{a_0} :
Assuming the exchange is in a steady state, the amount of energy emitted by Earth must equal the amount absorbed, and so:
\begin{align} 4\pi r_E^2 \sigma T_E^4 &= \pi r_E^2 \times E_{a_0} \\ &= \pi r_E^2 \times \frac{4\pi r_S^2\sigma T_S^4}{4\pi a_0^2} \\ \end{align}
TE can then be found:
\begin{align} T_E^4 &= \frac{r_S^2 T_S^4}{4 a_0^2} \\ T_E &= T_S \times \sqrt\frac{r_S}{2 a_0} \\ & = 5780 \; {\rm K} \times \sqrt{696 \times 10^{6} \; {\rm m} \over 2 \times 149.598 \times 10^{9} \; {\rm m} } \\ & \approx 279 \; {\rm K} \end{align}
where TS is the temperature of the Sun, rS the radius of the Sun, and a0 is the distance between the Earth and the Sun. This gives an effective temperature of 6 °C on the surface of the Earth, assuming that it perfectly absorbs all emission falling on it and has no atmosphere.

The Earth has an albedo of 0.3, meaning that 30% of the solar radiation that hits the planet gets scattered back into space without absorption. The effect of albedo on temperature can be approximated by assuming that the energy absorbed is multiplied by 0.7, but that the planet still radiates as a black body (the latter by definition of effective temperature, which is what we are calculating). This approximation reduces the temperature by a factor of 0.71/4, giving 255 K (−18 °C).[5][6]

However, long-wave radiation from the surface of the earth is partially absorbed and re-radiated back down by greenhouse gases, namely water vapor, carbon dioxide and methane.[7][8] Since the emissivity with greenhouse effect (weighted more in the longer wavelengths where the Earth radiates) is reduced more than the absorptivity (weighted more in the shorter wavelengths of the Sun's radiation) is reduced, the equilibrium temperature is higher than the simple black-body calculation estimates. As a result, the Earth's actual average surface temperature is about 288 K (15 °C), which is higher than the 255 K effective temperature, and even higher than the 279 K temperature that a black body would have.

Origination

Thermodynamic derivation of the energy density

The fact that the energy density of the box containing radiation is proportional to T^{4} can be derived using thermodynamics. It follows from the Maxwell stress tensor of classical electrodynamics that the radiation pressure p is related to the internal energy density u:

p = \frac{u}{3}.

From the fundamental thermodynamic relation


dU = T dS - p dV ,

we obtain the following expression, after dividing by dV and fixing T  :

\left(\frac{\partial U}{\partial V}\right)_{T} = T \left(\frac{\partial S}{\partial V}\right)_{T} - p = T \left(\frac{\partial p}{\partial T}\right)_{V} - p .

The last equality comes from the following Maxwell relation:

\left(\frac{\partial S}{\partial V}\right)_{T} = \left(\frac{\partial p}{\partial T}\right)_{V} .

From the definition of energy density it follows that

U = u V

where the energy density of radiation only depends on the temperature, therefore

\left(\frac{\partial U}{\partial V}\right)_{T} = u \left(\frac{\partial V}{\partial V}\right)_{T} = u .

Now, the equality

\left(\frac{\partial U}{\partial V}\right)_{T} = T \left(\frac{\partial p}{\partial T}\right)_{V} - p ,

after substitution of \left(\frac{\partial U}{\partial V}\right)_{T} and p for the corresponding expressions, can be written as

u = \frac{T}{3} \left(\frac{\partial u}{\partial T}\right)_{V} - \frac{u}{3} .

Since the partial derivative \left(\frac{\partial u}{\partial T}\right)_{V} can be expressed as a relationship between only u and T (if one isolates it on one side of the equality), the partial derivative can be replaced by the ordinary derivative. After separating the differentials the equality becomes

\frac{du}{4u} = \frac{dT}{T} ,

which leads immediately to u = A T^4 , with A as some constant of integration.

Stefan–Boltzmann's law in n-dimensional space

It can be shown that the radiation pressure in n-dimensional space is given by[10]

P=\frac{u}{n}

So in n-dimensional space,
dQ= dU + P dV\, =d(uV)+ \frac{u}{n} dV =V du + (\frac{n+1}{n})u dV
thus using 2 nd law of thermodynamics,we can write,
dS=\frac{Vdu}{T} + \frac{n+1}{n} \frac{u}{T}dV
Hence
\left(\frac{\partial S}{\partial u}\right)_{V}= \frac{V}{T}
and
\left(\frac{\partial S}{\partial V}\right)_{u}= \frac{n+1}{n} \frac{u}{T}
or
\left(\frac{\partial \frac{V}{T}}{\partial V}\right)= \frac{n+1}{n}\left(\frac{\partial \frac{u}{T}}{\partial T}\right)
yielding
\frac{1}{T}= \frac{n+1}{n}(\frac{1}{T}-\frac{u}{T^2}\frac{dT}{du})
or,
(n+1) \frac{dT}{T}= \frac{du}{u}
yielding,
u \propto T^{n+1}
implying
\frac{dQ}{dt} \propto T^{n+1}
The same result is obtained as the integral over frequency of Planck's law for n-dimensional space, albeit with a different value for the Stefan-Boltzmann constant at each dimension. In general the constant is
\sigma=\frac{1}{p(n)} \frac{\pi^{\frac{n}{2}}}{\Gamma(1+\frac{n}{2})} \frac{1}{c^{n-1}} \frac{n(n-1)}{h^{n}} k^{(n+1)} \Gamma(n+1) \zeta(n+1)[citation needed]
where \zeta(x) is Riemann's zeta function and p(n) is a certain function of n, with p(3)=4.

Derivation from Planck's law

The law can be derived by considering a small flat black body surface radiating out into a half-sphere. This derivation uses spherical coordinates, with φ as the zenith angle and θ as the azimuthal angle; and the small flat blackbody surface lies on the xy-plane, where φ = π/2.
The intensity of the light emitted from the blackbody surface is given by Planck's law :
I(\nu,T) =\frac{2 h\nu^{3}}{c^2}\frac{1}{ e^{\frac{h\nu}{kT}}-1}.
where
The quantity I(\nu,T) ~A ~d\nu ~d\Omega is the power radiated by a surface of area A through a solid angle dΩ in the frequency range between ν and ν + dν.

The Stefan–Boltzmann law gives the power emitted per unit area of the emitting body,
\frac{P}{A} = \int_0^\infty I(\nu,T) d\nu \int d\Omega \,
To derive the Stefan–Boltzmann law, we must integrate Ω over the half-sphere and integrate ν from 0 to ∞. Furthermore, because black bodies are Lambertian (i.e. they obey Lambert's cosine law), the intensity observed along the sphere will be the actual intensity times the cosine of the zenith angle φ, and in spherical coordinates, dΩ = sin(φ) dφ dθ.
\begin{align} \frac{P}{A} & = \int_0^\infty I(\nu,T) \, d\nu \int_0^{2\pi} \, d\theta \int_0^{\pi/2} \cos \phi \sin \phi \, d\phi \\ & = \pi \int_0^\infty I(\nu,T) \, d\nu \end{align}
Then we plug in for I:
\frac{P}{A} = \frac{2 \pi h}{c^2} \int_0^\infty \frac{\nu^3}{ e^{\frac{h\nu}{kT}}-1} d\nu \,
To do this integral, do a substitution,
u = \frac{h \nu}{k T} \,

du = \frac{h}{k T} \, d\nu
which gives:
\frac{P}{A} = \frac{2 \pi h }{c^2} \left(\frac{k T}{h} \right)^4 \int_0^\infty \frac{u^3}{ e^u - 1} \, du.
The integral on the right can be done in a number of ways (one is included in this article's appendix) – its answer is \frac{\pi^4}{15} , giving the result that, for a perfect blackbody surface:
j^\star = \sigma T^4 ~, ~~ \sigma = \frac{2 \pi^5 k^4 }{15 c^2 h^3} = \frac{\pi^2 k^4}{60 \hbar^3 c^2}.
Finally, this proof started out only considering a small flat surface. However, any differentiable surface can be approximated by a bunch of small flat surfaces. So long as the geometry of the surface does not cause the blackbody to reabsorb its own radiation, the total energy radiated is just the sum of the energies radiated by each surface; and the total surface area is just the sum of the areas of each surface—so this law holds for all convex blackbodies, too, so long as the surface has the same temperature throughout. The law extends to radiation from non-convex bodies by using the fact that the convex hull of a black body radiates as though it were itself a black body.

Appendix

In one of the above derivations, the following integral appeared:
J=\int_0^\infty \frac{x^{3}}{\exp\left(x\right)-1} \, dx = \Gamma(4)\,\mathrm{Li}_4(1) = 6\,\mathrm{Li}_4(1) = 6 \zeta(4)
where \mathrm{Li}_s(z) is the polylogarithm function and \zeta(z) is the Riemann zeta function. If the polylogarithm function and the Riemann zeta function are not available for calculation, there are a number of ways to do this integration; a simple one is given in the appendix of the Planck's law article. This appendix does the integral by contour integration. Consider the function:
f(k) = \int_0^\infty \frac{\sin\left(kx\right)}{\exp\left(x\right)-1} \, dx.
Using the Taylor expansion of the sine function, it should be evident that the coefficient of the k3 term would be exactly -J/6. By expanding both sides in powers of k, we see that J is minus 6 times the coefficient of k^3 of the series expansion of f(k). So, if we can find a closed form for f(k), its Taylor expansion will give J.

In turn, sin(x) is the imaginary part of eix, so we can restate this as:
f(k)=\lim_{\varepsilon\rightarrow 0}~\text{Im}~\int_\varepsilon^\infty \frac{\exp\left(ikx\right)}{\exp\left(x\right)-1} \, dx.
To evaluate the integral in this equation we consider the contour integral:
\oint_{C(\varepsilon, R)}\frac{\exp\left(ikz\right)}{\exp\left(z\right)-1} \, dz
where C(\varepsilon,R) is the contour from \varepsilon to R, then to R+2\pi i, then to \varepsilon+2\pi i, then we go to the point 2\pi i - \varepsilon i, avoiding the pole at 2\pi i by taking a clockwise quarter circle with radius \varepsilon and center 2\pi i. From there we go to \varepsilon i, and finally we return to \varepsilon, avoiding the pole at zero by taking a clockwise quarter circle with radius \varepsilon and center zero.


Integration contour

Because there are no poles in the integration contour we have:
\oint_{C(\varepsilon, R)}\frac{\exp\left(ikz\right)}{\exp\left(z\right)-1} \, dz = 0.
We now take the limit R\rightarrow\infty. In this limit the contribution from the segment from R to R+2\pi i tends to zero. Taking together the integrations over the segments from \varepsilon to R and from R+2\pi i to \varepsilon+2\pi i and using the fact that the integrations over clockwise quarter circles withradius \varepsilon about simple poles are given up to order \varepsilon by minus \textstyle \frac{i \pi}{2} times the residues at the poles we find:
\left[1-\exp\left(-2\pi k\right) \right]\int_\varepsilon^\infty \frac{\exp\left(ikx\right)}{\exp\left(x\right)-1} \, dx = i \int_\varepsilon^{2\pi-\varepsilon} \frac{\exp\left(-ky\right)}{\exp\left(iy\right)-1} \, dy + i\frac{\pi}{2}\left[1 + \exp \left(-2\pi k\right)\right] + \mathcal{O} \left(\varepsilon\right) \qquad \text{ (1)}


The left hand side is the sum of the integral from \varepsilon to R and from R+2 \pi i to 2 \pi i + \varepsilon. We can rewrite the integrand of the integral on the r.h.s. as follows:
\frac{1}{\exp\left(iy\right)-1} = \frac{\exp\left(-i\frac{y}{2}\right)}{\exp \left(i \frac{y}{2}\right) - \exp\left(-i\frac{y}{2}\right)} = \frac{1}{2i} \frac{\exp\left(-i\frac{y}{2}\right)}{\sin\left(\frac{y}{2}\right)}
If we now take the imaginary part of both sides of Eq. (1) and take the limit \varepsilon\rightarrow 0 we find:
f(k) = -\frac{1}{2k} + \frac{\pi}{2}\coth\left(\pi k\right)
after using the relation:
\coth\left(x\right) = \frac{1+\exp\left( -2x\right)}{1 - \exp\left( -2x \right)}.
Using that the series expansion of \coth(x) is given by:
\coth(x)= \frac{1}{x}+\frac{1}{3}x-\frac{1}{45}x^{3} + \cdots
we see that the coefficient of k^3 of the series expansion of f(k) is \textstyle -\frac{\pi^4}{90}. This then implies that \textstyle J = \frac{\pi^4}{15} and the result
j^\star = \frac{2\pi^5 k^4}{15 h^3 c^2} T^4
follows.

at

Friday, October 2, 2015

Homo naledi


From Wikipedia, the free encyclopedia

Homo naledi
Temporal range: not dated
Homo naledi skeletal specimens.jpg
A sample of the 1,550 skeletal pieces recovered

Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Family: Hominidae
Genus: Homo
Species: †H. naledi
Binomial name
Homo naledi
Berger et al., 2015
Rising Star Cave Gauteng South Africa location map.svg
Location of discovery in Guateng, South Africa

Homo naledi is an extinct species of hominin, provisionally assigned to the genus Homo. Discovered in 2013 and described in 2015, fossil skeletons were found in South Africa's Gauteng province, in the Dinaledi Chamber of the Rising Star Cave system, about 800 meters (0.5 miles) southwest of Swartkrans, part of the Cradle of Humankind World Heritage Site.[1][2] As of September 2015, fossils of at least fifteen individuals, amounting to 1550 specimens, have been excavated from the cave.[2]
The species is characterized by a body mass and stature similar to small-bodied human populations, a smaller endocranial volume similar to Australopithecus, and a skull shape similar to early Homo species. The skeletal anatomy combines primitive features known from australopithecines with features known from early hominins. The individuals show signs of having been deliberately disposed of within the cave near the time of death. The fossils have not yet been dated.[3]
Homo naledi was formally described in September 2015 by 47 co-authors proposing the bones represent a new species. Other experts contend more analysis and evidence is needed to support this classification.

Etymology

The word naledi means "star" in the Sotho language. It was chosen to correspond to the name of the Dinaledi chamber ("chamber of stars") of the Rising Star cave system where the fossils were found.[1]

Discovery


Illustration of the Dinaledi Chamber within Rising Star Cave, where bones proposed to be from H. naledi were excavated

In 2013 while exploring the Rising Star cave system, Rick Hunter and Steven Tucker found a narrow, vertically oriented "chimney" or "chute" measuring 12 m (39 ft) long with an average width of 20 cm (7.9 in).[2][4][5] Then Hunter discovered a room 30 m (98 ft) underground (Site U.W. 101, the Dinaledi Chamber), the surface of which was littered with fossil bones.The site and its fossils discovered on 13 September 2013 by Tucker and Hunter; before they entered the cave that day "the cavers knew that a scientist in Johannesburg was looking for bones";[5] on 1 October photos were shown "to Pedro Boshoff and then to Lee" Berger.[6] Hunter and Tucker are both members of Speleological Exploration Club of South Africa (SEC).

"It was clear from the arrangement of the bones that someone had already been there, perhaps decades before".[2]

"When the Dinaledi Chamber was first entered, the sediments along the cave floor (i.e., Unit 3) consisted largely of loosely packed, semi-moist, clay-rich clumps of varying sizes in which bone material was distributed. Where people had moved through the chamber, the sediment along the floor had been compacted down to a flat, semi-hard surface. The hominin bone material was distributed in Unit 3, across the surface in almost every area of the chamber, including narrow side passages and offshoots, with the highest concentration of bone material encountered near the southwest end of the chamber, about 10–12 m downslope from the entry point, where the floor levels out", according to Dirks et al. (2015).[4]

"There were bones everywhere. The cavers first thought they must be modern. They weren’t stone heavy, like most fossils, nor were they encased in stone—they were just lying about on the surface, as if someone had tossed them in. They noticed a piece of a lower jaw, with teeth intact; it looked human".[2]

Excavation and research

In November 2013, a twenty-one day excavation took place.[7] In March 2014 a three-week plus several days excavation was done in the Dinaledi Chamber uncovering 1,550 pieces of bone belonging to at least fifteen individuals, found within clay-rich sediments[8] by the National Geographic/University of the Witwatersrand Rising Star Expedition.[1] The layered distribution of the bones suggests that they had been deposited over a long time, perhaps centuries.[2] Only one square meter of the cave chamber has been excavated; other remains might still be there.[9][10][11]
Around "300 numbered bone fragments were collected from the surface of the Dinaledi Chamber, and ∼1250 numbered fossil specimens were recovered from" the chamber's excavation pit.[4] The fossils include skulls, jaws, ribs, teeth, bones of an almost complete foot, of a hand, and of an inner ear. The bones of old, young and infants were found.[2]

Dirks et al. say that ", in some of the material we observed, regions that conventionally disassociate both early and late in the decomposition process are represented as anatomically aligned and articulated groups, suggesting limited post-mortem spatial alteration and disaggregation within the chamber in both the proximate putrefactive (early) and distal decompositional (late) periods. Although much of the fossil material is disarticulated, the deposit contains articulated or near-articulated examples such as the maxilla and mandible of single individuals and the bones of the hands and feet, which normally disarticulate very early in the decomposition sequence (...). These elements are found in anatomical position and in spatial articulation with elements (e.g., vertebral components) that normally disarticulate later. The observed patterns indicate a formational process that did not involve a high degree of winnowing. The sedimentary system in which the fossils were deposited was closed, or nearly closed, and skeletal disarticulation and movement of elements was largely restricted to the Dinaledi Chamber only (although some bone fragments may have washed down floor drains)."[4]

In June 2014 a workshop ended that lasted several weeks; some say that it was the first of its kind within paleontology.

Some research articles "on the finds were submitted to the (...) journal Nature but were not published there";[12] a September 2015 Nature article said that the "team intends to publish at least a dozen papers from the workshop in coming months; the two published today [in another journal] are the first".[13]

Announcement

The fossils and the new species theory were announced in a news conference and ceremony on 10 September 2015 (in Johannesburg, South Africa) by Lee Berger and some members of the Rising Star Expedition.[1][2][14] Two research articles were published that day in the journal eLife – one proposing the new species,[1] and one describing the cave containing the fossils.[4] During the ceremony a display case of the fossils was unveiled.[15]

Fossils

Morphology

Berger et al. say that the physical characteristics of H. naledi has traits similar to the genus Australopithecus, mixed with traits more characteristic of the genus Homo, and traits not known in other hominin species.[1]

Adult males stood around 150 cm (5 ft) tall and weighed around 45 kg (100 lb), while females were a little shorter and weighed a little less. These sizes fall within the range of small-bodied modern humans. An analysis of H. naledi‍‍ '​‍s skeleton suggests it stood upright and was fully bipedal.[citation needed]

Its hip mechanics, the flared shape of the pelvis are similar to australopithecines, but its legs, feet and ankles are more similar to the genus Homo.[1][16]

"The proximal and intermediate manual phalanges are markedly curved, even to a greater degree than in any Australopithecus. The shoulders are configured largely like those of australopiths. The vertebrae are most similar to Pleistocene members of the genus Homo, whereas the ribcage is wide distally like Au. afarensis"[1] However, the species' brains were markedly smaller than modern Homo sapiens, measuring between 450 and 550 cm3 (27–34 cu in). Four skulls were discovered, thought to be two females and two males, with a cranial volume of 560 cm3 (34 cu in) for the males and 465 cm3 (28.4 cu in) for females, approximately half the volume of modern human skulls; average Homo erectus skulls are 900 cm3 (55 cu in). The H. naledi skulls are closer in cranial volume to australopithecine skulls.[2] Nonetheless, the cranial structure is described as more similar to those found in the genus Homo than to australopithecines, particularly in its slender features, and the presence of temporal and occipital bossing, and the fact that the skulls do not narrow in behind the eye-sockets.[1] The teeth and mandible musculature are much smaller than those of most australopithecines, which suggests a diet that did not require heavy mastication.[1] The teeth are small, similar to modern humans, but the third molar is larger than the other molars, similar to australopithecines.[16]

The hands of H. naledi appear to have been better suited for object manipulation than those of australopithecines.[1]

Some of the bones resemble modern human bones, and other bones are more primitive than the australopithecine, an early ancestor of humans. The thumb, wrist and palm bones are modern-like while the fingers are curved, more australopithecine, and useful for climbing.[2]

The overall anatomical structure of the species has prompted scientists to classify the species within the genus Homo, rather than within the genus Australopithecus. The H. naledi skeletons indicate that the origins of the genus Homo were complex and may be polyphyletic, and that the species may have evolved separately in different parts of Africa.[17] The arm has an Australopithecus-similar shoulder and fingers and a Homo-similar wrist and palm.[16] The structure of the upper body seems to have been more primitive than that of other members of the genus Homo, even apelike.[2]

A reconstruction of a model of a H. naledi head was made by measuring the bones of the head, the eye sockets, and where the jaw muscles insert to the skull. The measurements were used to make the model, including skin, eyes, and hair.[18]

Dating challenges

The fossils have not been dated as of 10 September 2015. The discovery team waited until after the research article was published before trying radiocarbon dating of the fossils because radiocarbon dating will have to destroy parts of the fossils.[19][20][21] Radiocarbon dating can only date fossils which are 50,000 or fewer years old, and can determine if the fossils are younger than 50,000 years old.[21]

Morphology is sometimes used to make some approximations about the temporal range of artifacts.[citation needed][how?] Geologists think the cave in which the fossils were discovered is no older than three million years.[22]

The bones were found lying on the cave floor or buried in shallow sediment. Two fossil dating techniques—dating fossils within volcanic ash by dating the ash, and dating fossils within layers of calcite flowstone deposited by running water by dating the flowstone—cannot be used because the fossils were not buried in volcanic ash or in flowstone layers.[2] For example: in East Africa, volcanic ash layers, which are datable, helped to determine the age of fossils like Lucy at 3.2 million years old; Berger himself used radiometric techniques to date his discovery of Australopithecus sediba bones found between two flowstone layers at another site.[2]

Berger said that the anatomy of H. naledi suggests it originated at or near the start of the Homo genus, around 2.5 million to 2.8 million years ago. The excavated bones may be younger.[23] Tim White says that it is hard to know if the fossils are "much less than one million years old" or older.[20]

Ownership

The University of the Witwatersrand is the curator of the fossils.[24] The fossils are owned by South Africa and will likely stay there (in accord with a 1998 resolution by the International Association for the Study of Human Paleontology – ratified also by South Africa – "strongly recommending that original hominid fossils not be transported beyond the boundaries of the country of origin, 'unless there are compelling scientific reasons which must include the demonstration that the proposed investigations cannot proceed in the forseeable future in the country of origin").[24]

Opinions

  • The research team proposes the bones represent a new species naledi in the genus Homo, other experts contend further analysis is needed to support this classification.[9][19]
  • Paleoanthropologist Tim D. White said the significance of this discovery is unknown until dating has been completed and additional anatomical comparison with previously known fossils has been done.[20]
  • Rick Potts said that without an age, "there's no way we can judge the evolutionary significance of this find."[23] "In terms of a combination of human and more primitive features, the volume of evidence from 15 individual skeletons is so compellingly different from anything that we've seen in other bipedal, upright human-like fossils that I'm completely convinced that it's a new species and part of our human evolutionary tree," H. naledi's teeth and skull are similar to early individuals of our genus, like Homo habilis. The feet are like those of later humans, as are parts the hands. "But it also has these long, curved fingers that indicate tree living behavior more than anything that we see in Australopithecus even" The raised shoulders and rib cage are like those seen in the Australopithecus group. "It has a combination of Australopithecus and Homo-like traits, so Berger and his team are guessing that it's related to the transition between those two groups, which was a time when different populations lived under varying survival pressures that led to very different evolutionary experiments and different combinations of Australopithecus and Homo traits in different areas across Africa." "But it's hard to know without a date whether it's from that period, as one of those experiments that then went nowhere, or whether it's in fact much less than one million years old. In that case, we could be talking about something that also didn't go anywhere and was just an isolated, probably very small population that persisted for a long time in splendid isolation."[20]
  • New York University anthropologist Susan Anton stated that even after dating, experts will likely spend many years striving to put these fossils in the proper context because there is no consensus in paleoanthropology about exactly how such comparisons are used to define the genus Homo. "Some would argue that striding bipedalism is a defining feature, so that being Homo means using a specific way of moving around the environment. Other scholars may look more to cranial characteristics as Homo family features."[20]
  • Bernard Wood, paleoanthropologist (George Washington University), agrees the remains represent a new species, but thinks the bones may represent a relic population that may have evolved in near isolation in South Africa, similar to another relic population, a small-brained species of Homo floresiensis from the island of Flores in Indonesia.[16]
  • With the number of individuals, and the sexes and age groups represented, scientists consider the find to be "the richest assemblage of associated fossil hominins ever discovered in Africa,[16] and aside from the Sima de los Huesos collection and later Neanderthal and modern human samples, it (the excavation site) has the most comprehensive representation of skeletal elements across the lifespan, and from multiple individuals, in the hominin fossil record."[1][6]

Claims of more than one species

Comparisons to H. erectus

  • Based on the published descriptions,  "From what is presented here, (the fossils) belong to a primitive Homo erectus, a species named in the 1800s," wrote Tim D. White in an email. "When you compare so-called H. naledi with the Homo skull SK 80/847 from the Swartkrans site 800 m (2,600 ft) away, you say wow, this looks awfully similar. This is what an early, small H. erectus looks like.", said Tim White.[20] White stated that the fossils could be considered to fall well within the variation of the species Homo erectus, and his concern was echoed by paleoanthropologist Eric Delson. Eric Delson, Lehman College, New York, who was not involved with the work, said his guess is that H. naledi fits within a known group of early Homo creatures from around two million years ago.[citation needed]
  • Anthropologist Chris Stringer wrote: "the small brain size, curved fingers and form of the shoulder, trunk and hip joint resemble the prehuman australopithecines and the early human species Homo habilis. Yet the wrist, hands, legs and feet look most like those of Neanderthals and modern humans. The teeth have some primitive features (such as increasing in size towards the back of the tooth row), but they are relatively small and simple, and set in lightly built jawbones. Overall, to my eye, the material looks most similar to the small-bodied examples of Homo erectus from Dmanisi in Georgia, which have been dated at ∼1.8 million years old".[17]
  • Berger rejected the possibility of the fossils representing H. erectus at the announcement news conference.[23]

Deliberate placement of bodies hypotheses

  • Anthropologist John D. Hawks, from the University of Wisconsin-Madison who was a member of the team, stated that the scientific facts are that all the bones recovered are hominid, except for those of one owl; there are no signs of predation, and there is no predator that accumulates only hominids this way; the bones did not accumulate there all at once. There is no evidence of rocks or sediment having dropped into the cave from any opening in the surface; no evidence of water flowing into the cave carrying the bones into the cave.[26] Hawks concluded that the best hypothesis is that the bodies were deliberately placed in the cave after death, by other members of the species.[27]
  • Dirks et al. say that "Mono-specific assemblages have been described from Tertiary and Mesozoic vertebrate fossil sites (...), linked to catastrophic events (...) Among deposits of non H. sapiens hominins, where evidence of catastrophic events is lacking, mono-specific assemblages have been associated typically with deliberate cultural deposition or burial". There is no evidence a catastrophe placed the bodies in the cave, the bodies were deliberately placed in cave.[4]
  • "Sedimentological and taphonomic descriptions of notable fossil sites indicate that fossils were trapped and preserved in caves as a result of a range of processes including death traps, scavenging, mud flows and predation. Distribution patterns of fossiliferous caves in the area suggest that fossil deposition occurred in caves that are close to critical resources such as water".[4]
  • William Jungers, chair of anatomical sciences at Stony Brook University" (...) doesn’t dispute that the H. naledi bones belong in the genus Homo and were likely deposited deliberately, but he cautions against "trying to argue for complex social organization and symbolic behaviors."
    There may be a simple answer. "Dumping conspecifics down a hole may be better than letting them decay around you." He suggests it’s possible that there was once another, easier, way to access the chamber where the bones were found. Until scientists can know the approximate age of the Homo naledi fossils, Jungers says they are "more curiosities than game changers. Intentional corpse disposal is a nice sound bite, but more spin than substance.""[7]
  • Carol Ward, professor of pathology and anatomical sciences (University of Missouri) and not directly involved in the study, is also skeptical of the intentional burial explanation and asked, "If it’s really that hard to get to the cave, how do you get to that long dark cave carrying your dead grandmother?"[28]

Ritual hypotheses

  • Berger et al. claim "these individuals were capable of ritual behaviour". They speculate the placing of dead bodies in the cave was a ritualistic behaviour, a sign of symbolic thought.[29] "Ritual" here means an intentional and repeated practice (disposing of dead bodies in the cave), and not implying any type of religious ritual.[22] Ritualistic behavior has been generally considered to have emerged among Homo sapiens and Homo neanderthalensis.[2] The oldest confirmed Neanderthal burial is 100,000 years ago.[26]
  • Potts describes it as more of a mystery: "There is no evidence of material culture, like tools, or any evidence any kind of symbolic ritual that we almost always associated with burial," he says. "These bodies seem to have simply been dropped down a hole and disposed of, and that really brings up a whodunit".[20]
  • Research article Dirks et al. (2015) says that "Every previously known case of cultural deposition has been attributed to species of the genus Homo with cranial capacities (brain size) near the modern human range. Unlike the Dinaledi assemblage, each of these hominin associated occurrences also contains at least some medium- to large-sized, non-hominin fauna".[4]
  • William Jungers is more dismissive of Berger’s suggestion that we may have inherited the practice of burying our dead from H. naledi, a creature with a much smaller brain than modern humans. "That’s crazy speculation—the suggestion that modern humans learned anything from these pin heads is funny."[7]
  • Berger thinks that deliberate disposal of bodies within the intricate cave system would have required the species members to find their way through total darkness and back again, and he speculates that this would have required light in the form of torches or fires lit at intervals.[2][30]

Politicians

South African President Jacob Zuma[31] and Deputy President Cyril Ramaphosa[32] congratulated the team that made the discovery.

2015 documentaries

A PBS NOVA National Geographic documentary Dawn of Humanity, describing the discovery of H. naledi, was posted online on 10 September 2015, and broadcast nationwide on 16 September 2015.[33] According to archeologist K. Kris Hirst, the Dawn of Humanity documentary film provides "a rich context for the discovery [of the fossils of Homo naledi], setting the historical and evolutionary background so that viewers can understand the significance of the discovery."[34]
The National Geographic Society has videos on its website describing, explaining and showing different phases of the discovery, the scientists, the six women researchers, excavation of the fossils during a two-year period, and the process of making a model of a head of H. naledi from the fossils.[35][36]

Gallery


Comparison of skull features of Homo naledi and other early human species.[17]
Fossil hand (palm and dorsum) of H. naledi
Fossil skull of H. naledi
Fossil foot of H. naledi – dorsal (A); medial (B); (C) arch – Scale = 10 cm (3.9 in)
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