Evolution and the hourglass
Today, 134 years ago, Darwin died. A suitable day to share a data visualization on evolution!
In the early 18-hundreds, Karl von Baer made a couple of observations that lead to what is now commonly known as Baer’s laws of embryology. These state that while embryos of various species look strikingly different in the beginning of embryo development and as adults, there is one time-point when the variation is at its minimum that typifies a phylum, the phylotypic stage. Baer’s observations were later developed further and became known as the developmental “hourglass” (Sander 1983?). *see FOOTNOTE*
The hourglass model states that there are developmental constraints that work against variation – but this lacked, as many evolutionary models, experimental validation. How should one recapitulate or test an experiment that in nature took billions of years? I fondly remember my teacher Ingo Wallat’s classes on evolution and was therefore delighted when joining Pavel Tomancak’s lab that a team around Alex Kalinka was collecting the first molecular proof for the developmental hourglass and van Baer’s 200-year old theory!
Their paper was published in 2010 (also available here), but I must admit the nature of the evidence was initially hard to grasp for a RNA biologist like myself! I therefore decided to create an illustration of their findings to explain the science to a wider audience – and maybe also high-school students!
* Haeckel beautifully illustrated a similar idea of his own, that embryonic development is a recapitulation of evolution. In fact, his drawings are most often used to illustrate the developmental hourglass – a great case point for the power of a wonderful scientific illustration!