How Old Are the Fossils?

“Chinese Unearth Fossil of 500,000-Year-Old Peking Man”

DO YOU see headlines like this from time to time? Maybe you wonder how they know the fossil is half a million years old.

There are several ways scientists estimate the age of fossils. The one in which they put highest confidence is the radioactive method. How does it work? Is it really accurate?

Usually it is not the fossil itself that is dated, but a radioactive mineral found in the same stratum of rock with the fossil.

URANIUM-LEAD DATING. Uranium is a radioactive element that very slowly changes into lead. The common form of uranium, U-238, disintegrates at such a rate that in 4,500 million years half of it changes into lead. The age of a mineral containing uranium can be determined by measuring how much lead has formed in it.

So from a chemical analysis of a mineral for its uranium and its lead content, a simple calculation gives its age. But the analysis is complicated by the fact that there are different isotopes of lead, and only lead 206 comes from uranium 238. So the chemist must get the help of the physicist with his mass spectrometer to see how much of this particular isotope is in the lead.

However, there are two very important assumptions that must be true if the answer is to be correct:

First, that there was no lead mixed in the uranium mineral when it formed in the cooling magma of molten rock. If there was any lead present, then the newly formed rock would look as if it was already millions of years old.

Second, that no lead has escaped from the mineral. If some of the telltale lead had been leached out of an old mineral, it would appear much younger under analysis.

So, you see, the method is not foolproof. Nevertheless, with proper attention to such possible pitfalls, acceptably reliable dates have been put on many old rock formations. Based on this method, the age of the oldest parts of the earth’s crust has been set at over four billion years.

But uranium minerals are not found in the same rocks as fossils. This is because in igneous rocks, or even those that have been metamorphosed by heat, any fossils would have been destroyed. So other radioactive clocks must be used for dating fossils.

POTASSIUM-ARGON DATING. The element potassium is widespread in the mineral world. It has a very rare isotope, K-40, which decays with a half-life of 1,300 million years. Most of it changes into calcium, but 11 percent of it decays in a different way, to argon. Now argon is an inert gas. It does not combine with other elements and is usually found only in the atmosphere. But minerals such as feldspar, containing potassium that has not been disturbed for a long time, do contain trapped argon because of the radioactive process.

This property of potassium is utilized in a situation where fossils have been buried in a fall of volcanic ash. The theory of dating by the potassium-argon method is simple. When a volcano erupts, the molten rock that is thrown out loses the argon that was previously formed from the potassium in the rock. The rock solidifies as the volcanic plume cools, and its potassium, now free of argon, starts over again making it. Thus the potassium-argon clock has been set to zero, and anything buried by the eruption can be dated by analyzing the surrounding ash.

The theory sounds good, but in practice difficulties arise once more in the basic assumptions. On the one hand, the possibility that argon has leaked out of the mineral would make the age measurement too small. On the other hand, if not all the argon was boiled out of the molten rock by the volcanic heat, the clock would be set in error at the beginning.

This can be especially serious in cases where the potassium-argon method is used on relatively recent deposits​—say, younger than a few million years. The slightest trace of argon remaining in the ash will cause a huge error. For example, if a potassium mineral had been buried, building up argon for a billion years before it was ejected in an eruption, then as little as one eighth of one percent of the argon left in the ash would date a freshly buried bone in it as being already a million years old.

This might not be a serious error in sediment a hundred million years old. But you can see how wrong it would make any claim for a supposed ancestor of man found in the Olduvai gorge in Tanzania​—a claim that the fossil is one or two million years old. It is hard to read seconds on a clock that has only an hour hand.

Corroborating the undependability of scientific dating, note the following. Two scientists wanted to relate a new find to a previous one, which had been dated as being 65 million years old. However, potassium-argon dating said their new find was only 44 million years old​—21 million less. No problem​—where there’s a will there’s a way. The two scientists “attribute this to loss of argon or to impurities,” reports Science News, July 18, 1981. Wishy-washy when it suits their purpose, dogmatic when it doesn’t.

RADIOCARBON DATING. The radiocarbon clock, based on a half-life of carbon 14 of 5,500 years, is much more useful for measuring ages in the span of man’s history on earth. In this case we are not using a radioactive element that has been here ever since creation. With such a short life, the radiocarbon would have all disappeared ages ago. But this isotope is being formed continually by the rain of cosmic rays upon the earth’s atmosphere.

All living things have carbon in their every body part, and while they are living they have the same proportion of carbon 14 as the carbon dioxide in the atmosphere. When they cease to live and are buried and cut off from the atmosphere, the carbon 14 gradually decays and disappears. So if an old piece of wood or charcoal is exhumed, one can measure the proportion of carbon 14 remaining and tell how long ago it was part of a living tree.

Again, that is the theory. In practice, there are many things that can cause false readings. One thing that can easily spoil a sample is possible contamination with other materials that might contain carbon either older or younger.

The most serious question, especially about very old specimens, is whether the radiocarbon was in the same proportion in the atmosphere in ancient times as it is today. There is no way to be sure of this, because it depends upon cosmic ray showers, which are notably variable and sporadic. If, for instance, for some reason during mankind’s earliest history, the cosmic rays averaged only half the intensity they have today, any sample from that era would appear to be 5,500 years older than it really is.

Since we have no way of knowing how intense cosmic rays were in past ages, we are wise to accept carbon-14 dates only for the period for which the clock has been calibrated with historical materials, back to about 3,500 years ago. Older than that, they may be increasingly inaccurate.

SO HOW CREDIBLE ARE THE DATES? Is the fossil Peking man really 500,000 years old? Let’s see what the Encyclopædia Britannica says about it. Speaking of matching fossils of similar animals in strata in different parts of the earth, it says:

“Such lines of evidence have led to the tentative conclusion that the species Homo erectus is essentially of early middle Pleistocene age. . . . the youngest accepted hard-core representatives of H. erectus in the fossil record would seem to be the group from Peking in China, Trinil in Java, Ternifine in Algeria, and the braincase of Olduvai hominid 9 from Tanzania. Repeated potassium-argon datings of the Trinil beds has yielded an estimate of their age in years as 550,000 BP (before present). . . . it would seem reasonable to suggest 1,500,000 to 500,000 BP as a time range for Homo erectus.”

Note all the hedging to avoid a definite assertion​—words such as “tentative,” “would seem,” “estimate,” “reasonable to suggest.” It is not stated that the Peking fossil has been dated. After a patchwork of inference, the conclusion ultimately stands on an analysis in which the retention in the potassium mineral of only a thousandth part of the argon that had previously accumulated could account for the whole 500,000 years. When we look behind the headlines we find no sound proof for the widely touted claims to antiquity of the Peking fossils.

If a person wants to find fault with the Bible history of man’s creation, he can use the contradictory claims of scientific dating methods to justify his position. But, to be fair, he really ought to acknowledge that such methods are too fallible and unreliable to challenge successfully the faith of one who accepts the Bible as God’s word of truth.

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Two very important assumptions must be true if the answer is to be correct

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It is hard to read seconds on a clock that has only an hour hand

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Wise to accept carbon-14 dates only when they can be calibrated with historical materials

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When we look behind the headlines we find no proof for the claims to antiquity of the Peking fossils