He began with Michael Behe and a short description of his irreducibly complexity argument regarding the bacterial flagellum--that since it fails to function if any piece is removed, and it's too complex to have originated by evolution in a single step, it must have been intelligently designed and created. He observed that 2,000 years ago, Galen made the same argument about the human hand and other aspects of human and animal anatomy. Galen wrote that "the mark of intelligent design is clear in those works in which the removal of any small component brings about the ruin of the whole."
Behe, Carrier said, hasn't done what you'd expect a scientist to do with respect to his theory. He hasn't looked at the genes that code the flagellum and tried to identify correlate genes in other microbes, for example.
In the ancient context, the debate was between those who argued for natural selection on random arrangements of features that were spontaneously generated, such as Anaxagoras and atomists like Democritus and Epicurus, vs. those who argued for some kind of intelligent design, like Plato, Aristotle, Cicero, and Galen. Carrier set the stage by describing a particular debate about the function of the kidneys between Asclepiades and Galen. Asclepiades thought that the kidneys were either superfluous, with urine forming directly in the bladder, or was an accidental sieve. Galen set out to test this with a public experiment on an anesthetized pig, which had been given water prior to the operation. He opened up the pig, ligated (tied knots in) its ureters, and they started to balloon and the bladder stayed empty. Squeezing the ureter failed to reverse the flow back into the kidney. When one ureter was cut, urine came out. Thus, Galen demonstrated that the kidneys extract urine from the blood and it is transported to the bladder by the ureters. The failure of the flow to operate in reverse showed that the kidneys were not simple sieves, but operated by some power that only allowed it to function in one direction. This, argued Galen, was demonstration of something too complex to have arisen by chance, and refuted the specific claims of Asclepiades.
Galen's 14-volume De Usu Portium (On the Usefulness of Parts) made similar arguments for intelligent design about all aspects of human anatomy--the nerve transport system, biomechanics of arm, hand, and leg movement, the precision of the vocal system, etc. He also asked questions like "How does a fetus know how to build itself?" He allowed for the possibility of some kind of tiny instructions present in the "seed," on analogy with a mechanical puppet theater, programmed with an arrangement of cogs, wheels, and ropes.
Galen also investigated the question of why eyebrows and eyelashes grow to a fixed length and no longer, and found that they grow from a piece of cartilage, the tarsal plate. He concluded that while his evidence required an intelligent designer, they entailed that God is limited and uses only available materials. Galen, a pagan, contrasted his view with that of Christians. For Christians, a pile of ashes could become a horse, because God could will anything to be the case. But for Galen, the evidence supported a God subject to the laws of physics, who was invisibly present but physically interacting to make things happen, and that God realizes the best possible world within constraints.
Which intelligent design theory better explains facts like the growth of horses from fetuses, the fact that fetuses sometimes come out wrong, and why we have complex bodies at all, rather than just willing things into existence via magic? If God can do anything, why wouldn't he just make us as "simple homogenous soul bodies that realize functions by direct will" (or "expedient polymorphism," to use Carrier's term)?
The difference between Galen's views and those of the Christians was that Galen thought of theology as a scientific theory that had to be adjusted according to facts, that facts about God are inferred from observations, and those facts entail either divine malice or a limited divinity. What we know about evolution today places even more limits on viable theories of divinity than in Galen's time. (Carrier gave a brief overview of evolution and in particular a very brief account of the evolution of the bacterial flagellum.)
Galen's views allowed him to investigate, conduct experiments to test the theories of his opponents as well as his own, and make contributions to human knowledge. He supported the scientific values of curiosity as a moral good, empiricism as the primary mode of discovery, and progress as both possible and valuable, while Christianity denigrated or opposes these. The views of early church fathers were such that once Christianity gained power, it not only put a halt to scientific progress, it caused significant losses of knowledge that had already been accumulated. (Carrier later gave many examples.)
Tertullian, a contemporary of Galen, asked, "What concern have I with the conceits of natural science?" and "Better not to know what God has not revealed than to know it from man."
Thales, from the 6th century B.C., was revered by pagans as the first natural scientist--he discovered the natural causes of eclipses, explained the universe as a system of natural causes, performed observations and developed geometry, made inquiries into useful methods, and subordinated theology to science. There was a story that he was so focused on studying the stars that he fell into a well. Tertullian wrote of this event that Thales had a "vain purpose" and that his fall into the well prefigured his fall into hell.
Lactantius, an early Christian writer and tutor of Constantine the Great, denied that the earth was round (as part of a minority faction of Christians at the time), said that only knowledge of good and evil is worthwhile, and argued that "natural science is superfluous, useless, and inane." This despite overwhelming evidence already accumulated of a round earth (lighthouses sinking below the horizon as seen from ships sailing away, astronomical observations of lunar eclipses starting at different times in different locations, the fact that different stars are visible at different latitudes, and the shadow of the earth on the moon), which Lactantius simply was uninterested in.
Eusebius, the first historian of the Christian church, said that all are agreed that only scriptural knowledge is worthwhile, anything contrary to scripture is false, and pursuing scientific explanations is to risk damnation. Armchair speculation in support of scripture, however, is good.
Amid factors such as the failure of the pagan system, civil wars in the Roman empire, and a great economic depression, Christianity came to a position of dominance and scientific research came to a halt from about the 4th century to the 12th-14th centuries.
Carrier compared these Christian views to specific displays at the Answers in Genesis Creation Museum in Kentucky, which compared "human reason" to "God's word." One contrasted Rene Descartes saying "I think therefore I am" to God saying "I am that I am." Galen wouldn't have put those into opposition with each other.
Another display labeled "The First Attack--Question God's Word" told the story of Satan tempting Adam to eat from the fruit of the tree of knowledge of good and evil, which highlights the "questioning" of Satan for criticism, and argues that putting reason first is Satanic.
Another diagram comparing "human reason" to "God's Word" showed evolution as a 14-billion-year winding snake-like shape, compared to the short and straight arrow of a 6,000-year creation.
Carrier noted, "It doesn't have to be that way. Galen's faith didn't condemn fundamental scientific values; Galen's creationism was science-based."
He then gave numerous examples of knowledge lost or ignored by Christianity--that Eratosthenes had calculated the size of the earth (a case described in Carl Sagan's "Cosmos" series), Ptolemy's projection cartography and system of latitude and longitude, developments in optics, hydrostatics, medicine, harmonics and acoustics, pneumatics, tidal theory, cometary theory, the precession of the stars, mathematics, robotics (cuckoo clocks, coin-operated vending machines for holy water and soap dispensing), machinery (water mills, water-powered saws and hammers, a bread-kneading machine), and so on. He described the Antikythera mechanism, an analog computer similar to WWI artillery computers, which was referred to in various ancient texts but had been dismissed by historians as impossible until this instance was actually found in 1900.
Another example was the Archimedes Codex, where Christians scraped the ink from the text and wrote hymns on it, and threw the rest away. The underlying writing has now been partially recovered thanks to modern technology, revealing that Archimedes performed remarkably advanced calculations about areas, volumes, and centers of gravity.
Carrier has a forthcoming book on the subject of this ancient science, called The Scientist in the Early Roman Empire.
A few interesting questions came up in the Q&A. The first question was about why early Christians didn't say anything about abortion. Carrier said it probably just wasn't on the radar, though abortion technology already existed in the form of mechanical devices for performing abortions and abortifacients. He also observed that the ancients knew the importance of cleanliness and antiseptics in medicine, while Jesus said that washing before you eat is a pointless ritual (Mark 7:1-20). Carrier asked, if Jesus was God, shouldn't he have known about the germ theory of disease?
Another question was whether Christianity was really solely responsible for 1,000 years of stangnation. Carrier pointed out that there was a difference between Byzantine and Western Christianity, with the former preserving works like those of Ptolemy without condemning them, but without building upon them. He said there are unerlying cultural, social, and historical factors that explain the differences, so it's not just the religion. He also pointed out that there was a lost sect of Christianity that was pro-science, but we have nothing of what they wrote, only references to them by Tertullian, criticizing them for supporting Thales, Galen, and so forth.
Another questioner asked how he accounts for cases of Christians who have contributed to science, such as Kepler, Boyle, Newton, and Bacon. Carrier said "Not all Christians have to be that way--there's no intrinsic reason Christianity has to be that way." But, he said, if you put fact before authority, scripture will likely end up not impressing you, being contradicted by evidence you find, and unless you completely retool Christianity, you'll likely abandon it. Opposition to scientific values is necessary to preserve Christianity as it is; putting weight on authority and scripture leads to the anti-science position as a method of preservation of the dogma.
It was a wonderfully interesting and wide-ranging talk. He covered a lot more specifics than I've described here. If you find that Carrier is giving a talk in your area, I highly recommend that you go hear him speak.
You can find more information about Richard Carrier at his web site.