The scheme of Figure 6-3 represents a slighty different view: it does accept that some primitive eukaryotes left modern descendants – and this qualifies them as a true kingdom – but also accepts that some archezoa could have lost organelles secondarily, and are not therefore true paleokaryota.
The most important information of Figure 6-3, however, does not concern the number or the names of taxonomic kingdoms, but the distinction between extinct kingdoms (gray boxes) and surviving kingdoms (white boxes). The data provided by molecular phylogenies are all obtained from living organisms, i.e. from representatives of surviving kingdoms, and clearly they will never give us direct information about extinct kingdoms. If we use only molecular phylogenies, in other words, we are bound to conclude that extinct kingdoms did not exist, and that the first living cells were eubacteria.
In order to have a less biased reconstruction of the early history of life, therefore, the data from molecular phylogenies must be integrated by theoretical considerations. Those data are absolutely necessary, no doubt about that, but we should not forget that they are not sufficient. The history of life on earth does not coincide with the history of the molecules that did survive, as if extinct kingdoms never existed. Molecules, therefore, take us back in time but only to a certain point. Beyond that, only our theories can make the journey.

Three thousand million years

Let us imagine that an extraterrestrial civilization wanted to study our planet’s life and decided to send a spaceship on Earth once every million years. For at least 2000 times, the answer would have always been the same: “the dry lands are completely sterile, and in the seas there are only diluted colonies of bacteria”. After that, the despatches would have been slightly different: “now there are little amounts of oxygen in the atmosphere, and the seas are also inhabited by bigger cells which have a nucleus”. This verdict would have been repeated for about another 1000 times, and the extraterrestrials would have had any right to conclude that life’s evolution on Earth was exasperatingly slow.