Evolution in the News - July 2006
by Do-While Jones

It’s About Time

The techniques scientists use to determine age generally depend upon measuring very small quantities. A small measurement error (or a small amount of contamination) will significantly affect the calculations. In the case of carbon 14 dating, we can check the carbon 14 dates against historic dates to determine their accuracy. But what about techniques that claim to be able to determine dates millions of years in the past? How do you check them for accuracy?

In practice, dates are checked against evolutionary prejudice. If they confirm the theory of evolutution, they are correct. If they don’t, they are wrong.

Perhaps you remember the controversy last summer over footprints first discovered in a quarry south of Mexico City in 2003.

Scientists have long believed that the first humans came to North America after the last Ice Age ended about 13,500 years ago. According to that theory, they crossed a land bridge from Asia into what is now Alaska and spread quickly across the continent.

The theory is supported by the stone tools they left behind -- all less than 13,500 years old. Their tool technology was named "Clovis'' for the New Mexico town where it was first described. 1

Thomas Higham of the University of Oxford, UK, used radiocarbon dating on shells in sediments just above the layer of ash [containing the footprints] and found they were about 40,000 years old.

Early arrival

The prevailing theory is that people first migrated from northern Asia between 15,000 and 10,000 years ago, crossing to America over a land bridge at the Bering Strait. But controversial genetic analyses of Native American populations indicate that some immigrants may have arrived much earlier than that, up to 40,000 years ago. 2

Notice that carbon 14 dating of the layer just above the footprints yielded a date of 40,000 years before present, and some people were reluctant to believe it. But people who had done genetic analysis embraced the date immediately because it confirmed their theory. Which date do you believe? The one based on the age of the stone tools, or the one based on carbon 14?

But that was last July. Scientists investigated some more.

A report of human footprints preserved in 40,000-year-old volcanic ash near Puebla, Mexico (http://www.royalsoc.ac.uk/exhibit.asp?id=3616&tip=1), was the subject of a press conference that stirred international media attention. If the claims (http://www.mexicanfootprints.co.uk) of Gonzalez et al. are valid, prevailing theories about the timing of human migration into the Americas would need significant revision. Here we show by 40Ar/39Ar dating and corroborating palaeomagnetic data that the basaltic tuff on which the purported footprints are found is 1.30 +/- 0.03 million years old. We conclude that either hominid migration into the Americas occurred very much earlier than previously believed, or that the features in question were not made by humans on recently erupted ash. 3

If a scientist believes the ash is 1.3 million years old, he won’t try carbon 14 dating because there won’t be any carbon 14 left to measure. Any carbon 14 found, must be the result of contamination, yielding a date that is too young. On the other hand, if the scientist believes the ash is only 40,000 years old, he won’t try argon dating because “excess argon” contamination will yield a date that is too old.

Ah, but the argon date is confirmed by “corroborating paleomagnetic data.”

The palaeomagnetic analysis (Fig. 2) reveals two components of essentially opposite polarity, a reverse component held by magnetite within the lapilli and an antipodal normal component held by goethite within the claylike matrix. The sample is azimuthally unoriented but the stratigraphic top and bottom are known. The reverse polarity component held by the tuff is thermoremanent, which strongly suggests that the tuff pre-dates the Brunhes/Matuyama geomagnetic polarity transition at about 790 kyr (ref. 2). A reverse polarity is consistent with magnetization acquired during chron C1r.2r, between 1.07 and 1.77 Myr (ref. 3), as indicated by the 40Ar/39Ar data. Because only the in situ top of the sample is known, the declination is arbitrary but the inclination (-32.1°) is meaningful and compares favourably with that (-33.4°) expected from a reversed geocentric axial dipole at this latitude. 4

Paleomagnetic analysis is based on the fact that rocks are very, very, very weakly magnetized. The belief is that when the rocks were still liquid, just before they hardened, their minerals aligned themselves with the weak magnetic field of the Earth. Furthermore, scientists “know” that the Earth’s magnetic field has reversed itself many times, and that one of those times when it was reversed was 1.3 million years ago.

Nobody knows why the magnetic field reverses, and nobody can explain how life would have survived that period of time between reversals when the Earth had no magnetic field to protect it from cosmic radiation, but still, some people believe it happened, and exactly when it happened. How do they know that? They “know” it from the type of the fossils found in the rocks, and the amount of time it took those fossils to evolve.

Pictures of the footprints show that individual prints look just like human footprints, and they are found in a left-right pattern just like humans make when walking. But an irrational belief in the accuracy of argon and paleomagnetic dating, and a prejudicial belief in when humans appeared in North America, has led some scientists to say that they aren’t really human footprints, even though they just look a just like footprints.

Humans are not thought to have even been around 1.3 million years ago. According to most scientific estimates, modern humans didn't begin appearing in Africa until about 200,000 years ago. If the markings really are footprints, then it would mean one of two things: either humans appeared much earlier than previously thought or the footprints were made by an early ancestor of humans like homo erectus.

Renne thinks both possibilities are extremely unlikely. So where does that leave things?

Man, cow or machine

After visiting the site, Renne believes the markings are not really human footprints at all, but rather impressions left by machines or animals that have passed through the quarry in recent times.

"You have to remember this is a public area," Renne said in a telephone interview. "Vehicles drive across it, you can see tire tracks on the surface. There are cows and other animals grazing nearby." 5

He thinks this rock formation, which has been dated at 13,000 years old by the tools it contains, 40,000 years old by carbon 14, and 1.3 million years old by argon dating, is really so young that modern men, cows, or machines have made footprints in it. He doesn’t believe any of the “scientific” dating methods because they don’t agree with his evolutionary prejudice.

The unthinkable conclusion is that that radiometric dating doesn’t work. But the scientific literature is filled with contradictory radiometric dates. Here are just a few examples from articles we collected during the past year alone.

One aspect of early Earth evolution that remains puzzling is the discrepancy between the more rapid formation time of the core given by the decay of hafnium to tungsten than by the clock based on the decay of uranium to lead. 6

One clock relies on the rate at which hafnium-182 radioactively decays into tungsten-182. By that reckoning, the core was formed 30 million years after the origin of the solar system, or about 4.54 billion years ago. But a second clock, based on the decay of two isotopes of uranium into lead, dates the core to 80 million years after the solar system’s birth. 7

The use of hafnium-tungsten chronometry to date the Moon is hampered by cosmogenic tungsten-182 production mainly by neutron capture of tantalum-181. 8

Second, the terrestrial inventory of the isotope 40Ar created by radioactive decay of 40K leaves too much of this gas unaccounted for. 9

Dating of the rocks had suggested they were roughly 180 million years old. However, the red planet shows no widespread signs of having been bombarded that recently. 10

The bottom line is that, if you look long enough, you can find a dating technique that will tell you what you want to hear (in a particular circumstance, if you make the “correct” assumptions). If so, then that dating technique must be reliable. But that same dating technique might be unreliable in another circumstance, if it tells you what you don’t want to hear.

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Footnotes:

1 Associated Press, July 5, 2005, “Footprints in Mexico Create Scientific Stir” (Ev)
2 Dalton, Nature, Published online: 4 July 2005, “Ancient 'footprints' found in Mexico” (Ev)
3 Renne et al., Nature 438, 1 December 2005, “Geochronology: Age of Mexican ash with alleged 'footprints'” (Ev)
4 ibid.
5 Than, LiveScience posted: 30 November 2005 “Controversial Footprints: Earliest Man or Modern Machine?” (Ev)
6 Natue, 27 October 2005, “Beat the Clock” page xi, (Ev)
7 Natural History, February 2006, “The Earth Gets Clocked”, page 18, (Ev)
8 Science, 9 December 2005, “Hf-W Chronometry of Lunar Metals and the Age and Early Differentiation of the Moon”, page 1671, (Ev)
9 Science, 16 December 2005, “Helium Feels the Heat in Earth’s Mantle”, page 1777, (Ev)
10 Nature, 8 December 2005, “Rocks’ clocks reset”, page 715, (Ev)