In the 1930s, the Land Bridge Theory was our best explanation for how people settled North America. Logically-speaking the theory was strong yet it lacked the ability to be effectively tested. Theories incapable of being either confirmed or proven false (falsified) aren't particularly useful. A theory which cannot be tested is no theory; it is pure speculation, a guess. A good theory is one which can be used to make predictions; and if a prediction is confirmed it suggests the theory in question is approximately true. The more evidence we acquire the more trustworthy the theory. Concrete evidence in support of the Land Bridge Theory came in 1949 when chemist Willard Libby introduced the world to radiocarbon dating. Radiocarbon dating was the first reliable technique of determining the absolute age of carbon-based (organic) objects. After radiocarbon tests were conducted on archaeological sites throughout North America, science established that the oldest sites of human settlement were on the West Coast (just as the theory predicted).
Scientists predicted the earliest settlements were to be found on the West Coast because the climate there was comparatively more hospitable than that of the interior. Predictions like these are developed through a combination of logic and evidence; that is, experience and reality dictate that no one capable of good judgment would settle, let alone survive, in too harsh of a climate. Another piece of evidence favoring a coastal settlement hypothesis was the fact that during the last ice age North America's interior was not only too cold to support people but also unreachable because of glaciers. By contrast people living in coastal settlements could take advantage of both the abundance of fish/shellfish but also the climate made livable by the warm ocean air. Based on all of this scientists assumed that as the glaciers receded people populated the remainder of North, Central and South America.
If the Land Bridge Theory had a weakness it was probably the assumption that the Western Hemisphere was only ever settled once by humans in the entire history of the Earth. If this were the case, and there was only one migration, scientists predicted Indians currently living in North America would be genetically similar to people living in Siberia (the source of the migration 13,000 years ago). In the 1970s, improvements in our understanding of genetics allowed us to finally test this prediction. The test results revealed Siberians and North America's indigenous peoples did in fact belong to the same haplogroups; that is, they shared a common ancestor in the not too distant past. The one migration hypothesis seemed to be getting stronger and stronger with every subsequent piece of evidence.
In his book The Grand Design, the theoretical physicist Stephen Hawking observed scientists prefer theories that are simple and elegant over ones which are un-necessarily complex. At first glance the Land Bridge Theory seems to fit Hawking's requirements—it is simple, straightforward, and supported by multiple lines of evidence. So what could possibly be wrong with such a theory? Well, a theory is ultimately only as strong as the evidence in support of it; and since scientists do not know everything they frequently have to make (logical) assumptions; and if experts do not have all the information the assumptions they make might lead them to make mistakes or miss important details. Despite the problem of not having all the information the scientific method is still nonetheless a powerful tool of discovery. This is because when new evidence emerges we improve our theories by changing them to take in to account the new information. Some critics of science point out that since we keep making mistakes and scientific knowledge is continually changing science is inherently untrustworthy. People who hold such a view do not understand the scientific method very well; that is, in many respects science works on the basis of trial and error, i.e. you try one thing and it doesn't work so you try another, etc. In the end, when scientists admit they've made mistakes they use what they've learned to refine their theories making them more accurate (which most sane people would admit is a good thing). But to refine a theory you need more and better information. Scientists uncover this information using one or both of two available tools, e.g. experimentation and/or logical conjecture.
The benefit of experiments are obvious, but what about logical conjecture? What is logical conjecture exactly? Why is it useful? And are there any problems with it?
What is logical conjecture?
We use logical conjecture whenever we need to reach a conclusion without necessarily having all the required information. Logical conjecture is not quite the same thing as an educated guess. This is because when you use it you aren't guessing at all. Instead, you are using the available evidence (you have to have some actual evidence to start from) to construct a picture of what probably happened, what is probably happening or what will happen.
For example, consider the following situation: while walking in an alley you notice a broken window in the apartment building to your right. You also see shards of glass and a baseball on the ground. If I were just guessing I might jump to a conclusion like the window broke because of a loud sound. Although it is not impossible sound was responsible for the glass breaking it is not supported by the evidence.
When using logical conjecture you do not guess: you begin with what evidence you have (a ball, shards of glass nearby) and move towards making a conclusion like the ball was thrown and probably broke the window. This explanation, compared to the sound breaking the window, is simpler and more likely. But is it the only possible explanation? No. Yet, it appears to be the only one supported by the evidence available.
Why is logical conjecture useful?
Logical conjecture is useful for helping us organizing small pieces of evidence in to a 'bigger picture"; to put it simply it allows us to complete a puzzle even though many pieces of the puzzle are missing.
For example, in 1924 astrophysicist Edwin Hubble observed in his telescope that some galaxies appeared to be "blue-ish" while others appeared "red-ish". Hubble was confused by what he saw; however, he knew that both light and sound traveled as waves. In the case of sound, waves sounded different based on whether or not the sound was moving away as opposed to towards an observer. This well-known phenomenon is called the Doppler Effect. Hubble used his understanding of sound and extrapolated that light also behaved as though it had a Doppler Effect; that is, when an object moved away from an observer it cast red light while objects moving towards the observer cast blue. Physicists call this phenomenon "red shift".
In 1927, astronomer Henri Joseph Édouard Lemaître built on the work of Hubble. He conjectured that since galaxies were moving away from one another that at some time in the distant past they must have been closer together, even occupying a single point in space (13.82 billion years ago according to the most current scholarship in 2013). To put it another way: the fact galaxies were moving away from us implied that space was expanding.
The idea of an expanding universe is an idea most of us find reasonable or take for granted nowadays. Yet, prior to Lemaître's work the vast majority of astrophysicists (including Albert Einstein) believed the universe was in fact static, immovable, and set in place "as is". Lemaître's work led to the eventual development of the Big Bang Theory. The Big Bang Theory is all fine and good, but what does it have to do with our discussion regarding logical conjecture? Well, we cannot actually go back in to the past and see the Big Bang occurring. Instead, we have to use what evidence is available and from that deduce that space is expanding. Again, logical conjecture is not guesswork; it is the disciplined use of logic and evidence to arrive at conclusions.
Are there any problems with logical conjecture?
No method of reasoning or evidence gathering is perfect. Again, a theory is only as good as the evidence you have acquired in support of it. If you are missing critical pieces of information your theory will not be as accurate. Such theories might even lead you to make false assumptions about reality.
For example, let's return our attention to the alley (see picture to the right). When we formed our first hypothesis, e.g. the ball broke the window, etc. we weren't aware of several other pieces of evidence. When we examined the scene more closely we discovered that a cat was hiding behind a garbage can where we also found a broken pot and plant beside it. This new evidence requires we revisit our original hypothesis.
Specifically, our assumption that the ball broke the window is now not the only reasonable explanation for what has happened. Based on this new information we would be justified in forming a second hypothesis, e.g. the cat knocked the plant through the window above. Which hypothesis is correct? Until both hypotheses are tested we don't actually know anything. Really, these hypotheses are only useful starting points to begin an investigation.
The biggest problem with logical conjecture is that in order to use it we have to make assumptions (there really is no other choice). The assumptions we make are based on the evidence we have. If we do not have all the information, we might form a "big picture" but that picture will be incomplete; nevertheless, it would be an error to assume that just because a picture is incomplete that it is not useful. For example, our understanding of particle physics has improved over the past century of study; however, we do not know everything there is to know about this field. Yet, we are still developing some pretty interesting technologies using the Standard Model of physics, e.g. GPS, space travel, satellites, etc. Nonetheless, some humility is in order; that is, when we are too confident in the explanatory power of a theory we risk blinding ourselves to how we might either improve our understanding or see how our understanding may be flawed.
The Land Bridge Theory and the Problem of Expectations
Scientists, just like everyone else, inherit assumptions from their parent culture. The parent culture shapes how we look at the world. In the previous section, I mentioned how most people simply take it for granted that the Big Bang is a fact. Why do the majority of people accept it as true? Based on the evidence? Perhaps. Though to be honest I'm not so sure. The theory itself is simple enough to grasp but how many people are actually acquainted with the Big Bang's supporting evidence like "red shift" or the "cosmic background radiation"? In my honest opinion, the theory has been around for so long, and is so well-established in the parent culture, most of us just grow up believing (rather than understanding) the Big Bang is an accurate description of the universe's origins.
Scientists are not perfect. They can and do make mistakes. They also take steps to prevent mistakes from happening in the first place. They design rigorous experiments; they keep one another honest. Our assumptions, many of which we just unquestioningly inherit from our parent culture, make us see the world as determined or incapable of novelty. For example, according to the original Land Bridge Theory there was only ever one migration of people from Asia to North America. Only one. As of the early 1990s, the evidence we had in our possession supported the one-migration view.
But what are scholars saying about it today? Can we still learn anything new about how the Western Hemisphere was populated by humankind? I guarantee we have not searched every inch of land. There must still be important discoveries to make. Therefore, it seems to me to be incredibly likely we'll discover archaeological sites older than 13,000 years of age. Sometimes when we think we've answered a question once and for all, or when we stop asking meaningful questions, we grow too confident in our theories. And that confidence can get us in to trouble. This is because when theories cease being accepted on the basis of evidence alone, and instead become something simply believed in, the affected scientist becomes less willing or able of conceiving of alternative explanations. For example, if I believe there was only one migration event then the evidence of a settlement (dated to 20,000 BCE) will either be missed or ignored. You see expectation blinds us to the possibility that reality is different than what we thought; and when we believe in theories too strongly we become biased and incapable of seeing the forest for the trees.
I am sure some of you have found the concepts I've introduced you to difficult at times to understand. Thinking is hard work. You just need to be patient with yourself and never stop asking questions. To quote the industrialist Henry Ford, "Thinking is the hardest work there is, which is probably why so few engage in it." So let's leave science behind for a moment shall we? Let's look at the problem of expectations by drawing upon an example from pop culture. Click here to watch a very important video starring ME! (Well, not really.)
In 2013, scientists in France discovered a giant virus which they named the Pandoravirus. The Pandoravirus is extraordinary for two reasons: firstly, it has 2,556 genes whereas other viruses only have approximately 1,000; and secondly, of the 2,556 genes discovered we only have actually seen 200 of them before. This means about 2,300 genes are completely new to science. Scientists themselves wondered why it took them so long to discover this massive virus. Some experts suggested that they’d probably seen it on many occasions; however, the virus wasn't recognized because of its size; it was likely mistaken for being a form of much larger bacteria. The Pandoravirus provides an excellent example of how expectations can make us complacent and prevent us from discovering new truths.
The Land Bridge Theory is Questioned
If you took a history or social studies class in North America over the past several decades, it is likely to assume you've learned some form of one or both of the following ideas:
1). Before the arrival of Columbus in 1492 AD the "New World" was an untamed wilderness.
The sparse populations who lived on this virtually empty land were unremarkable civilizations.
Now if I were taught that two ideas represented an accurate description of the settling of North America the one migration hypothesis becomes a little easier to believe. Yet, practicing good science requires we do not become beholden or enthralled to any one idea. We must be free to question anything and everything. Obviously, it is wise to go where the evidence happens to take us; however, it is also wise to remain skeptical of even our best established theories. The importance of skepticism cannot be over-stated; that is, being skeptical or questioning established wisdom is the only way we progress in our knowledge or understanding. Speaking of skepticism...
The Land Bridge Theory started coming under fire in 1997 when an archaeological dig in Southern Chile (see Chinchihuapi Creek at Monte Verde) revealed compelling evidence of human habitation much earlier than 13,000 thousand years ago. Scientists have struggled to reconcile this new information with the Land Bridge Theory as it was first understood. According to the theory this ancient Chilean civilization should not have even existed.
Yet, exist it did. Scientists have conjectured that the early settlers of Chile must have made the journey southward before the ice-free corridor ever appeared (if it ever appeared—new evidence has emerged even placing the existence of ice-free corridor idea in to doubt).
In his book 1491, science writer Charles C. Mann observed that "[given] the near impossibility of surpassing the glaciers without the corridor, some archaeologists suggested the first Americans must have arrived twenty thousand years ago, when the ice pack was smaller. Or even earlier than that—the Chilean site had suggestive evidence of artifacts more than thirty thousand years old. Or perhaps the first Indians traveled by boat, and didn't need the land bridge. Or maybe they arrived via Australia, passing the South Pole (Mann, Page 19)."
Although confidence in the Land Bridge Theory has been shaken by new evidence the model itself has not been discarded. Despite the theory's apparent shortcomings it nonetheless still has explanatory power, e.g. we can still use it to describe the most recent migration to North America. However, the theory is changing to accommodate the new findings like the archaeological sites in Chile and elsewhere. Again, to quote Charles Mann, "Scientists do not necessarily agree on all of the details; some scientists have theorized that the Americas may have been hit with as many as five waves of settlement before Columbus, with the earliest occurring as much as fifty thousand years ago. In many versions, though, today's Indians are seen [comparatively speaking] as relative latecomers (Mann, Page 19)."
SOURCE: Mann, Charles. 1491: New Revelations of the Americas Before Columbus. New York: Vintage Books, 2005.