A common critique aimed at mainstream science is based on drawing a sharp distinction between “observational” and “historical” science. Observational science is presented as more objective and much less worldview-dependent than historical science. This distinction is employed by young-earth creationists (YECs) in attempts to show that they accept science, while still rejecting the scientific consensus on natural history. However, as many have pointed out before me, “observational” vs. “historical” science is a misleading distinction. Hypotheses about the past can generate predictions that can be tested against observations in the present. And conversely, present-day observations and experiments spur new hypotheses about the past. In this way, observation, hypothesis, and prediction exist in a perpetual “feedback loop,” giving us greater and greater clarity about the course of natural history.
In this post, I discuss four remarkable examples of scientific hypotheses about the past that have been confirmed in remarkable ways by contemporary observations, supporting the scientific consensus on natural history.
Astronomy: Cosmic radiation and the Big Bang
Soon after Einstein laid out his theory of general relativity, cosmologists realized the equations implied we are living in an expanding universe. It led to the formulation of the Big Bang model, which has received confirmation from observations in a variety of ways ever since. Besides direct measurements of the universal expansion, a very notable confirmation of the Big Bang model is the Cosmic Microwave Background (CMB). Cosmologists reasoned that, if the cosmos has been expanding, there must have been an era in the past when it was too dense for light to travel freely. All matter was so hot and densely packed that protons and electrons were unable to form stable hydrogen atoms. These free charges were constantly interacting with the available light rays. Because of this constant scattering, the light was in balance with the energy of the hot material which filled our baby universe. But as the universe kept expanding, a time came (estimated ~379,000 years after the singularity) when the universe was cold enough for protons and electrons to combine and form stable hydrogen atoms. The multitude of light rays could suddenly start traveling freely, without being scattered all the time. As a result, an intense flash of light was released at that point in cosmic history. It was released everywhere, producing a bath of radiation that permeates the entire cosmos. This cosmic radiation reflects the properties of the universe back then, extremely hot and almost uniform.
In 1948, cosmologists estimated the properties of this hypothesized cosmic radiation. They knew from observations and theory at which temperatures protons and electrons are able to form stable hydrogen atoms: below about 3,000 degrees Kelvin. Given the thermal equilibrium of the universe at that temperature, they predicted that the emitted radiation would follow almost perfectly a blackbody spectrum (see image below). Over 13 billion years later, the expansion of the universe would have stretched out the radiation by a factor of 1000. This would reduce the temperature by the same factor, down to about 3 degrees Kelvin. After stretching, the typical wavelength of light ends up in the microwave region (millimeters), hence the name: Cosmic Microwave Background. When this prediction was made, no instruments were yet available to test this prediction.
17 years later, in 1965, the prediction was confirmed accidentally by radio astronomers Penzias and Wilson. They were trying to measure radio signals bouncing off weather balloons. However, their measurements were disturbed by background noise which appeared to be coming from all directions. After going over their equipment many times, they excluded the possibility of any technical faults. It puzzled them greatly. Then, by mere chance, Penzias learned that a group of astrophysicists over at Princeton had been making preparations to look for cosmic radiation as a remnant of the Big Bang. It was at this point that Penzias and Wilson started to come to terms with the significance of their discovery. Their observations were within the exact predicted range of the Princeton astrophysicists, and fit the predicted shape of the spectrum (resulting in a Nobel Prize; see the image below).
Furthermore, cosmologists reasoned that the CMB should also vary slightly in temperature across the sky. Such fluctuations would have resulted from the first “seeds” of density in the early universe, from which galaxies eventually formed. However, ground-based telescopes were not sensitive enough to measure such small differences. Another 27 years later, in 1992, a space telescope (the Cosmic Background Explorer or COBE) was used to directly test for these extremely tiny variations in the temperature of the CMB. Indeed, they were observed as predicted (resulting in another Nobel Prize).
ABOVE: The “brightness” (actually flux density in MJy per steradian) of the CMB plotted as a function of wavelength (in cm). Image created by author using publicly available data from NASA.
Geology: The Chicxulub Crater
The geologic column is a record of Earth’s history. Anywhere on the Earth, the rock layers tell the story of the events that led to Earth’s present condition. One of these layers is called the K-Pg boundary, a thin line of rock separating the Cretaceous period (145-66 million years ago) from the Paleogene period (66-43 million years ago). The K-Pg boundary consistently dates to an age of 66 million years, as shown by radiometric dating methods. Below this layer, dinosaur fossils are frequently found. Above it, dinosaur fossils are completely absent. This has been interpreted as evidence of a global extinction event around that time, that decimated the dinosaurs.
In 1980, a team of researchers led by physicist Luis Alvarez discovered extremely high iridium concentrations at the K-Pg boundary, present all over the planet. Iridium is extremely rare in the crust of our planet and is known to be abundant in asteroids and comets. Therefore, Alvarez and his colleagues proposed that the iridium layer resulted from a massive asteroid-impact at the time of the K-Pg boundary. If the collision was large enough, it could have led to dramatic changes in the earth’s climate, making it unsuitable for large cold-blooded animals like dinosaurs. In turn, that could explain the observed absence of dinosaurs above the K-Pg boundary.
Alvarez and colleagues also roughly estimated the size of the crater left behind by this impact: about 250 km in diameter. When this idea was proposed, there was no documented crater that matched the event. This was not extremely problematic, because geological processes can erase craters over time. However, in 1990 geologists identified a massive crater buried under the Chicxulub region on the coast of Yucatan, Mexico. The crater was dated to the the same age as K-Pg boundary and it has an average diameter of about 180 km, close to the size calculated by the Alvarez team! As a result, a massive asteroid-impact at that location is now widely believed to be one of the major causes of the extinction of dinosaurs.
In the Origin of Species, Darwin laid out his case for the common descent of all life. Accordingly, he predicted that intermediate varieties must have existed for all major evolutionary transitions. He also explained the apparent rarity of actually observing such transitional species in the fossil record:
“But just in proportion as this process of extermination has acted on an enormous scale, so must the number of intermediate varieties, which have formerly existed on the earth, be truly enormous. Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and gravest objection which can be urged against my theory. The explanation lies, as I believe, in the extreme imperfection of the geological record” (280).
Over the decades since Darwin’s work was published, paleontologists have expanded their record of fossil layers and classified fossilized organisms down to the finest detail. Amazingly, along the way, many of the predicted transitional species have been showing up in fossil form. For example, Darwin’s theory predicts a transitional form that bridges the gap between archaic fish and the first amphibious creatures (ancestors of most land creatures). It is a fish-like creature with limb-like fins (with arm-like bones) that would have allowed it to navigate land. It also had a neck and rib cage, allowing for breathing like amphibians, in addition to small gill slits that formed the basis for the development of ears in later species.
Paleontologist Neil Shubin was determined to find this fossil. He made a prediction about where the fossil would be found: in rock layers dated to around 375 million years old. This age marks the transition in the geological record below which only sea creatures are found, and above which amphibians are present.
In 2004, Shubin discovered a remarkable specimen that exactly represents such a transition: the Tiktaalik. Amazingly, it was found in exactly the rock layer where Shubin had predicted. In other words, Tiktaalik was found precisely where the transition would have taken place if Darwin’s ideas about common descent were correct.
ABOVE: Lobe-finned fish such as Eusthenopteron (bottom left) are followed by multiple adaptations in the fossil record: Panderichthys (center left), adapted for muddy shallows; Tiktaalik (top center) with limb-like fins that could take it onto land; Early four-legged animals adapted for weed-filled swamps, such as Acanthostega (center right; with feet with digits) and Ichthyostega (top right; with limbs). Lobe-finned fish such as Coelacanth (bottom right) also descended from the Eusthenopteron. Image source
Paleobiogeography: Genes and Geography of Marsupials
In the age of genetics, new ways of interlocking evidence have become available. By measuring the genetic “distance” between closely related species and mutation rates, it is possible to estimate how long ago these species diverged from each other. Using this “molecular clock,” we can build a very detailed tree of all life (see image below). For significant genetic differences to arise between groups of animals within a species, they need to become isolated from each other to some extent. Such isolation can be ecological (different niches within one environment) and geographical (different locations). Eventually, through genetic drift and natural selection, such sub-groups can become different species altogether. One very direct cause of geographical isolation is continental drift. As entire continents gradually drift apart, sub-populations on either side of the rift are deprived of the opportunity to mate with each other. In this way, the geological history of our planet should have left traces in the genes of present-day species.
A very notable example of geology affecting genetics is found in the clade of marsupials, mammals with pouches. Marsupials have a different kind of reproduction system than placental mammals and are only ever found in the Americas and in Australia. Marsupials constitute an entirely separate group of mammals with unique features (e.g. koalas, rat-like species, wombats, and kangaroos.). See the image below for an impression of the genetic “family tree” of Marsupials. Using the present-day drift of continents and other geological evidence, the separation between Antarctica and Australia was estimated to have occurred over the period of 50-100 million years ago. The clade of marsupials encompasses land animals that are currently present on both sides of the rift.
Therefore, biologists expected to find evidence of this geological separation in the genes of marsupials, with the molecular clock dating to the same time period. And this is exactly what they have observed in the genetic “history” of marsupials. In particular, the species monito del monte (a tree-mouse; part of Microbiotheria below) is only found in South America today and in the fossil record of Antarctica and Australia. Based on genetic divergence, its lineage is estimated to have separated from the Australian marsupials 75 million years ago, coinciding with the geological separation of Antarctica and Australia. It is amazing that two entirely separate fields of study, geology and genetics, both with completely different methods, converge on a unified picture of natural history.
ABOVE: The clade of marsupials. All present-day marsupials descended from a founding population of ancestral marsupials (on the left). The green lines then indicate the branching “family tree” (derived from genetic divergence). All Australian marsupials fall under the superorder Australidelphia, as do the Microbiotheria. The only surviving species of Microbiotheria is the Monito del Monte, which diverged from the Australian Marsupials 75 million years ago. Geographical locations are noted, with the Americas and Antarctica in blue and Australia in red, which were separated 50-100 million years ago, indicated with the red dashed line.