For extraterrestrial life, astrophysicist, science writer and coauthor Mario Livio gives a fair-minded and thorough review. He focusses on geochemistry and planetary histories, and his skepticism about Viking and ALH84001 includes insights from both sides. He favors robots over manned planetary missions, but shows no awareness of the crinoid fossil photographed by the Opportunity Mars rover. Both authors treat panspermia as an interesting distraction.

The book is extremely well researched, has plenty to teach and is easy to read. But let me comment on the optimistic origin-of-life message. While well-funded research continues, the gap between what is observed about the workings of cellular life and what can be explained by mainstream science continually grows. The conviction that life must originate is almost never recognized for what it is — an unexamined assumption. Meanwhile, all of the direct evidence says that life comes only from life. Since that is so, the assertion that life can originate actually constitutes an extraordinary claim. By Carl Sagan's dictum, there should be extraordinary evidence.

If we admit the possibility of life with no origin, other questions can be fruitfully explored.

The relative contribution and timing of different mechanisms that expanded the genome between FECA and LECA are illustrated, loosely based on the branch length-based estimates from single gene trees. They comprise the contributions to both the nuclear and mitochondrial genomes. The red bar indicates the gain of proto-mitochondrial genes upon endosymbiosis. The timing estimates of de novo gene inventions correspond to a minimal estimate of their age based on the earliest duplications they underwent. The black line illustrates the total number of genes in the composite genome, including only those genes that were still present in LECA. The loss of proto-mitochondrial and FECA genes before LECA cannot be timed. EGT, endosymbiotic gene transfer; HGT, horizontal gene transfer.

To comment: the evidence shows that eukaryogenesis relies heavily on transferred genes. The de novo ones also likely came by transfer (what else?) How genes transferred from prokaryotes or viruses acquired their eukaryotic programming is a huge, unanswered question. The puzzle is at least equal for evolution strictly among prokaryotes, where the reliance on transfer is seen to be complete. None of it makes neo-darwinian sense. A radical rethink is needed.

The heart of the book is its very close look at more than a hundred images from four Mars rovers, Opportunity, Spirit, Phoenix and Curiosity. Forms that, on Earth, would certainly be recognized as stromatolites, get close scrutiny and reasoned argument. Various other forms of microbial life are also convincingly visualized in the Mars images, with helpful comments. The discussion of bluberries and "newberries" is unconventional and enlightening.

Skeptics will likely compare some of their suggestions to the sighting of canals on Mars by Giovanni Schiaparelli, later championed by Percival Lowell. That would ignore the careful quantitative and statistical analysis also provided. The subject deserves to be considered by the broadest range of experts, especially sedimentologists, with open minds and genuine curiosity. This book is an excellent new resource for the question of past life on Mars.