There has been one rule that evolutionary biologists felt they could cling to: the amount of complexity in the living world has always been on the increase. Now even that is in doubt.
The Tree of Life
In recent years, genetic analysis has forced biologists to consider the possibility that organisms such as the sea squirt might have lost some of the complexity of their ancestors. Yet even now, few recognise the full implications of loss as a key player in evolution. The entire tree of life has been built on the assumption that evolution entails increasing complexity. So, for example, if two groups of animals were considered close because both had a particular prominent feature, then someone discovered a third, intermediate line that lacked that feature but shared many other aspects of the two groups, traditional phylogenists would conclude that the feature had arisen independently in the two outlying groups, by a process known as convergent evolution.
They often did not even consider the alternative explanation: that the feature in question had evolved just once in an ancestor of all three groups, and had subsequently been lost in the intermediate one. Now a handful of molecular biologists are considering that possibility.
Instead of simply looking to see whether two species share certain genes, the new approach involves taking the "molecular fingerprint" of different types of cells. It identifies the unique combination of transcription factors - molecules that control which of a cell's genes are turned on and when - that specify the make-up of a cell, including the molecular signals it transmits and receives.
If two groups of organisms share the same type of cells, with the same molecular fingerprint, giving rise to similar features in both, then it is extremely unlikely that these features evolved twice. So any intermediate groups of organisms that lack that feature would most likely have lost it during the course of evolution. Only now, with the ability to explore at the molecular level how morphological features have been lost, gained and modified over time, is the true extent of evolutionary loss coming to light.
Finding: In genetic variation - an ancestor may develop certain traits, only to be followed by generations of child variants that lose the trait, and then redevelop the trait.