After discussing orbital forcing of climate and how we can judge its effects in the geological record, I thought I should return to talking about hominins, but not without a smooth segue between the two. Two more proxies can also indirectly tell us what ancient climate was like, and these are are isotopes of carbon, C, and the metal strontium, Sr.
This global time-travelling adventure starts in the town of Monterey, California where an outcrop of an oil-rich rock formation caught the interest of a bunch of researchers. Oil is the remains of ancient diatoms (a group of algae) which have been under very high pressure for millions of years, and the layers of sediment in the rock itself also record the amount of algae that is present in the ocean at any one time. The layers of the Monterey Formation which formed 20 – 15 million years ago have particularly high levels of these algae. These simple plants take in carbon dioxide from the atmosphere via photsynthesis, and their subsequent death and deposition on the sea floor locks this carbon away (Raymo, 1994).
Two questions arise here: what causes these great algal blooms? And: what do carbon and strontium isotopes have to do with it?
Between 21 and 17 million years ago the Himalayas in Tibet were under great stress as India was continuing to plough into mainland Asia, and the great pressures generated literally squeezed rock like toothpaste from deep underground onto the surface in a process known as extrusion. Monsoon rains then increased because the greater amount of land heats up more and draws in more moist air from the sea, while taller mountains force the air to rise further. This rising air cools, the water vapour condenses out and falls as rain (fig. 1). This is a monsoon weather system, with rainfall caused by high ground called relief rainfall or orographic rainfall (Ruddiman, 2008).
The Pacific Ocean
Such rainfall would have a great weathering effect on the newly exposed rocks of the Himalayas, eroding minerals at an extremely high rate and carrying them down rivers and eventually into the Pacific Ocean where the dissolved nutrients can encourage massive algal blooms.
There are other ways nutrients could have entered the surface waters of the Pacific, such as a change in ocean circulation patterns, but high levels of strontium-87 (87Sr) as found in Himalayan rock are also seen in the foram record from the same time period, where strontium replaces calcium in the shell (Edmond, 1992; Raymo, 1994).
The other stable isotope of strontium is 86Sr but, unlike 16O and 18O (see last week), both 87Sr and 86Sr are equally likely to be included in foram shells. This means that fluctuations in δ87Sr directly reflect seawater δ87Sr, which in turn is directly related to river output and erosion.
Which brings us to carbon, specifically 12C and 13C. These are the two stable isotopes of carbon that are present in nature, with 12C constituting 98.89% of all carbon and 13C making up 1.1% (Wikipedia). The remaining tiny percentage is the slightly radioactive 14C. Like most plants, algae capture light and carbon dioxide to build sugars using a form of photosynthesis which discriminates very strongly against the heavier 13C, meaning that the rocks of the Monterey formation have a very low δ13C.
Another form of photosynthesis uses a slightly different mechanism to build sugars from light and carbon dioxide which doesn’t discriminate against 13C, and is seen in grasses which dominate the African savanna and sedges which are more often found in wetland environments. This is where a famous fossil, nicknamed Nutcracker Man for quite some time, comes into the story.
Nutcracker man or, more scientifically, Australopithecus boisei, is a robust australopithecine which lived across east Africa between 2.3 and 1.4 million years ago with important fossils coming from Tanzania and Ethiopia.
Au. boisei wasn’t the only robust asutralopith in Africa around that time, and superficial similarities in cranial form between Au. boisei and South African Au. robustus (fig. 3) was taken to mean that both species had similar diets. Scott et al. (2005) report that microscopic wear patterns on the surface of Au. robustus’s teeth created by food items demonstrated that its diet consisted mainly of very hard and brittle food items, and that extremely well developed jaw musculature could produce the high forces required to eat these items, while the large teeth (fig. 4) with very thick enamel reduced the risk of damage.
United States of America
Looking more closely at the teeth, researchers lead by Thure Cerling at the University of Utah discovered that the enamel had an unusually high δ13C value which could only be achieved during tooth development if Au. boisei was feeding not on hard nuts and fruit but on grasses or sedges (Thure et al., 2011) which would have contributed between 75% and 80% of dietary intake (Lee-Thorp, 2011).
Here, again, we can turn to δ18O to shed some more light on the situation. The incredibly low δ18O values of Au. boisei enamel are only really comparable to the hippopotamus, indicating that wetland sedges were likely a very important component in the diet of Australopithecus boisei. This work also agrees well with the wear patterns observed on Au. boisei teeth which lack cracks and pits caused by hard object fracture (Ungar & Sponheimer, 2011).
For the robust australopiths it seems that the hyper-robust cranial skeleton represented similar, or convergent, adaptation as a solution to two different dietary challenges. For Au. boisei in east Africa, its large jaw muscles allowed it to chew for long periods of time in order to process flexible but very tough vegetation without fatigue, while similarly highly developed musculature in South African Au. robustus allowed it to generate high forces to fracture hard and brittle food items.
So there we have it. Things are not always as they seem, Au. boisei is the “Human” Cow, the very similar Au. robustus is indeed still Nutcracker Man, and this story is another win for the scientific process of question and analysis.
As ever, if there’s anything I could have said better please feel free to ask questions in the comments or on Tw*tter.
p.s. For more information on the anatomy of robust australopiths, check out their entry in my Upright Walking series of posts.
Edmond, J. (1992). Himalayan tectonics, weathering processes, and the strontium isotope record in marine limestones. Science, 258, 1594–1597. Freely available here.
Lee-Thorp, J. (2011). The demise of “Nutcracker Man”. Proceedings of the National Academy of Sciences of the United States of America, 108, 9319–20. doi:10.1073/pnas.1105808108. Freely available.
Newswise, 2008. Early humans from east Africa were equipped to dine on hard foods but preferred a softer fare. Charlottesville, VA. Available from: www.newswise.com. Accessed: 30/10/14.
Raymo, M. (1994). The Himalayas, organic carbon burial, and climate in the Miocene. Paleoceanography, 9, 399–404.
Ruddiman, W. (2008). Earth’s Climate: Past and Future, 2nd Edition, W. H. Freeman and Company.
Ungar, P. S., & Sponheimer, M. (2011). The diets of early hominins. Science, 334, 190–3. doi:10.1126/science.1207701. Freely available here.
Scott, R. S., Ungar, P. S., Bergstrom, T. S., Brown, C. A., Grine, F. E., Teaford, M. F., & Walker, A. (2005). Dental microwear texture analysis shows within-species diet variability in fossil hominins. Nature, 436(7051), 693–5. doi:10.1038/nature03822
Cerling, T. E., Mbua, E., Kirera, F. M., Manthi, F. K., Grine, F. E., Leakey, M. G., … Uno, K. T. (2011). Diet of Paranthropus boisei in the early Pleistocene of East Africa. Proceedings of the National Academy of Sciences of the United States of America, 108, 9337–41. doi:10.1073/pnas.1104627108. Freely available.