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In Brief July 2009

JMcCFirst steps


Geoscientist 19.7 July 2009


Bipedality is a key human adaptation that first appeared in the fossil record about six million years ago. Preservation of our ancestors’ footprints in the East African Rift Valley is unfortunately fragmentary, and there has been much disagreement about its interpretation; although everyone accepts that modern Homo sapiens footprints contrast strongly with those of African apes during quadrupedal and bipedal locomotion.

In 1978, Mary Leakey1 found 3.7 million year-old (Pliocene) footprints at Laetolil in the Tanzanian extension of the Rift valley south from Kenya. These primitive prints are thought to have been made by Australopithecus afarensis. Though a few other footprints dated about 1.5 million years old were reported from Kenya on the east shore of Lake Turkana about this time2, only recently has a set of prints of this age (1.53-1.51 million years), well-enough preserved to make significant comparisons with the Laetolil and modern human prints, been discovered3 at Ileret, north of Koobi Fora.

The prints occur in the Okote member of the Koobi Fora Formation. There are sets of prints on a 9m-thick sequence of thin, graded silt and sand units, and the prints are sandwiched between two layers of fluvially reworked volcanic ash, firmly dated at 1.51.and 1.52 Ma. The prints lie at two stratigraphic levels, the upper with three hominin trails (two of two prints and one of seven), and the lower, 5m below, with one trail of two prints and a single print. (There are actually three volcanic ash horizons and the upper, Noth Ileret Tuff, also shows evidence of hominin activity.)

Footprint The prints are similar to those discovered in 1981 in the Akait Tuff, 45km to the south2, dated at 1.435Ma. They show a deeply depressed and adducted (clenched) big toe, typical of modern human footprints. They were made by an organism with longer lower limbs than the Laetolil footprints, attributed to Australopithecus australensis.
Differing from modern human footprints, their size is consistent with Homo ergaster/Homo erectus, who had shorter arms and legs, and a more “modern” foot and gait than A. afarensis. Increased mobility opened up a wider range of potential habitats. (Laetolil prints are radically different, since the australopithecines could not balance the upright body and lacked the postural adjustments effected by humans.)

Some on the Web have questioned the conclusions, including a suggestion that the prints belonged to gorillas. But the circumstantial evidence of their close geographic and geological association with the famous Hominin site east of Lake Turkana seems strong, and the evidence of hominin activity nearby would seem to support the conclusions of this excellent addition to the literature on the Rift Valley - which continues to reveal new discoveries since I mapped there more than 50 years ago.

References

  1. Leakey, M.D., Hay, R.L. 1979. Pioneer footprints in the Laetolil beds at laetolil, northern Tanzania. Nature 278; 317-323.
  2. Behrensmeyer, A.K., Laporte,L.F. 1981. Footprints of a Pleistocene hominid in northern Kenya. Nature 289; 167-169.
  3. Bennett, M.R., Harris, J.W.K., Richmond, B.G. et al. 2009. Early hominin foot morphology based on 1.5 m.y.old footprints form Ileret, Kenya. Science 323; 1197-1201.
  4. Leakey, M.D., Hay, R.L. 1979. Nature 278; 317.

MercuryVisitors from Mercury?


There has been speculation from time to time whether certain meteorites emanate form the planet Mercury. There has been very little publication on whether large scale impacts onto Mercury could reach our planet. Gladman and Coffey1 have recently covered this topic admirably. A small minority of the total meteorite population is reliably attributed to impact ejection from Mars: there is no proof of this but gaseous inclusions within them match the Martian atmosphere in composition and it is a case of applying ‘Occam’s razor’, they must emanate form a planet and Venus’s atmosphere would prevent ejection from there. True, there remains the small, innermost planet, Mercury, with a surface revealed by Mariner 10 to be pitted with impact craters; but recent imagery has also revealed many volcanic features (Geoscientists passim.).

The launch speed needed to escape the gravity of Mercury is intermediate between that of the Moon and of Mars, so escape is feasible. This is the only planet where impact speeds routinely exceed the escape speed by 5 to 20 times, so many impacting objects will escape its gravitational pull, a large fraction achieving heliocentric orbit. These authors conclude that the time taken for delivery on Earth would be ~30 Myr, and several percent or the material ejected from Mercury would reach Earth (slightly less than from Mars, and the same amount would reach Venus).

Since all the material ejected is Mercury-crossing in its orbit, much of it would be re-accreted. This is of little significance to the feasibility of Earth delivery, but is of interest when considering the possibility of impact-stripping of the mantle of the proto-Mercurian planet by a giant collision, recently reviewed4. This suggestion has been advanced to explain the high density of Mercury, and its postulated large core. Gladman and Coffey conclude that this stripping could only operate if the ejected mantle fragments were small enough to be dragged into the Sun on the timescale of a few million years, otherwise they would be re-accreted.

As to Earth delivery of Mercury-source meteorites, this is clearly a reality, although we do not know anything of their composition and petrology, and at present suggestions as to this remain in the realm of speculation. Somewhere among the vast accumulation of more than 30,000 meteorites known to science, there are hidden Mercury-sourced objects? Detecting them will be a challenge for the future!

References

  1. Gladman, B, Coffey, J. 2009. Mercurian impact ejecta: Meteorites and the Mantle. Meteoritics & Planetary Science 44(2); 285-291.
  2. McCall, G.J.H. 2008. Spider is a caldera. Geocientist 18 (5); 6.
  3. McCall, G.J.H. 2006. A caldera volcano of Brobdingnagian scale: Olympus Mons. Geooscientist 16(4); 29-30
  4. Benz,W., Anic,A, Homer, J., Whitby,J. 2007. The origin of Mercury. Space Science Reviews 132; 189-202.