A fundamental tenet of Christian belief is that God created the world, with the primary evidence for this found in the Book of Genesis. What is usually meant by Creationism is not so much about whether this is read literally or symbolically, but about creating a "science" that can be put alongside the science of evolution and therefore defend the biblical account.
'Young Earth Creationism'
Traditional creationism, often known as ‘Young-earth Creationism’ holds that the world was created in just six days, and only a few thousand years ago. Many Young-earth Creationists do not insist on the earth having been created barely six thousand years ago, but they feel certain it was created within the last ten thousand years. Of course, this means that much of what is taught in science is to be regarded as wrong.
One of the most implausible scientific hypotheses of Young Earth Creationists is that the half-lives of radioactive elements altered dramatically and permanently as a result of the biblical Flood, thus leading to false scientific estimates of the age of the earth. The limitation here is that this can only ever be a hypothesis and can not be used experimentally to falsify the accepted age of the earth. As it happens, this has been falsified by the evidence that extra-terrestrial elements have exactly the same half-lives as do terrestrial elements, but there is no evidence that the YEC proponents anticipated this or claimed their hypothesis to be falsifiable.
'Old Earth Creationism'
Old-earth Creationists accept that science has proven its case, that the world is over four billion years old. They seek to harmonise this with the Bible by looking for innovative ways of interpreting the Book of Genesis, to allow both science and the Bible to be seen as true. In other words, there is no overt sense of demonstrating the age of the earth or the evolution of species to be falsifiable, as long as God guided the process. What is regarded as falsifiable is that evolution progressed by natural selection.
Unusual among Creationists is Michael J. Behe, because he is a scientist and actually understands scientific method and has proposed a scientifically falsifiable thesis of biological origins. Behe believes that complexity in the structure of the biochemical system can not be explained by evolution. He evokes images of immensely complex biochemical systems, claiming that the complexity of these systems is irreducible and therefore could not be the result of an evolutionary process, pointing to intelligent design. Among his examples are the mechanisms of blood clotting, the cilium mechanism used by bacteria to move around, and the human eye. This is a falsifiable thesis because if these mechanisms are not irreducibly complex, with no simpler mechanisms from which they could have evolved, then his thesis of irreducible complexity is falsified. In fact, each of these examples has been examined by critics and examples from which evolution by natural selection could have occurred have been found by the scientific method.
Somewhat similar to Old Earth Creationism is 'theological evolution'. Because it adopts the scientific consensus, with a theologiccal component, this theory of origin is not scientifically falsifiable, except to the extent that a major body of science is falsifiable. Pope Francis, in a speech given at a Pontifical Academy of Sciences, stated:
“[God] created beings and allowed them to develop according to the
internal laws that he gave to each one, so that they were able to
develop and to arrive and their fullness of being. He gave autonomy to
the beings of the universe at the same time at which he assured them
of his continuous presence, giving being to every reality. And so
creation continued for centuries and centuries, millennia and
millennia, until it became which we know today, precisely because God
is not a demiurge or a magician, but the creator who gives being to
all things... The Big Bang, which nowadays is posited as the origin of
the world, does not contradict the divine act of creating, but rather
requires it. The evolution of nature does not contrast with the notion
of creation, as evolution presupposes the creation of beings that