Because the evidence supporting it is so strong, scientists no longer question whether biological evolution has occurred and is continuing to occur. In that respect, the past and continuing occurrence of evolution is a scientific fact. However, scientists also use the term "fact" to refer to a scientific explanation that has been tested and confirmed so many times that there is no longer a compelling reason to keep testing it or looking for additional examples. In science, a "fact" typically refers to an observation, measurement, or other form of evidence that can be expected to occur the same way under similar circumstances. As shown below, their fossil record clearly suggests that several lineages underwent increases and decreases and then (once again) increases in segment number over the course of millions of years. They are dated to more than 500 million years ago. They lived an amazingly long time ago (relatively speaking, anyways). These are creatures that are in the same clade as modern insects and crustaceans. In fact, we know such reversals have taken place. Case in point, additional segments can evolve, and this development may later reverse. Rather, everything is a blended mess, a cloudy soup of organisms that share (and do not share) certain traits at a number of different junctures.įor example, lineages can change quickly or slowly, and it can move forwards or backwards. We're also not looking at a chain, where there are clear stops along the track, and there are no clear branches that we can look to in order to trace our progress forwards and our evolution upwards. What's Wrong With the Ladder/Tree Metaphor < iframe src="" scrolling="no" frameborder="0" marginheight="0" marginwidth="0" style="width:1px height:1px display:none "> < /iframe> įirst, we are not looking at rungs on a ladder, where we can clearly see what came before and what comes next. If we must use a metaphor, a spider's web may be a better image-a twisted, chaotic mass of diverging lines and broken wisps the radiate outwards. B) A tree that has its roots planted firmly in the mud-puddles that birthed the first organisms, while its branches extend out to hold all of the modern, more advanced organisms.īut evolution is neither a ladder nor a tree. To this extent, according to this logic, evolution should look like either A) A ladder, with the first species resting at the bottom and subsequent, more complex, more evolved species climbing ever higher and becoming more developed.
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