Landward bound

 

that organic evolution on earth began first in the sea is certain, but the manner in which land was colonised by biota has largely remained in the realm of conjecture. The two earlier books on this subject were Colin Little's The Colonization of Land (1983) and Terrestrial Invasion (1990). The book under review discusses the problem of transition of biota from sea to land by taking advantage of the micro-palaeontological and molecular approaches. There-fore, it represents a synthesis between palaeobiology and neobiology. The authors also take into account the geological, palaeo-climato logical, palaeo-ecological and palaeoontological approaches.

The transition of biota from sea to land has been a major step in the history of evolutionary biology. Once it was achieved, the earth provided a vast habitat for colonisation by and the evolution of newer land-based micro-organisms, plants and animals.

The transition among plants involved desiccation due to the deficiency of water and nutrients on land, the transport of water, stomatal functioning, change in the length of the day and the products of photosynthesis, and a new reproductive biology. The direct result of this was the evolution of a vascular system in plants for water conduction and mechanical support, stomatal regulation for gas exchange and growth, and reproduction on land.

Animals managed the transition by using water and air as respiratory media, thermal exchange and adequate body hydration. Other factors that assisted the shift from sea to land were the abundance of inorganic solute on land, the production and disposal of metabolic wastes, different life history patterns and sensory modalities. The waste produced by one species became food for the other so that natural systems were left with almost no waste.

Perhaps the first organisms to 'invade' land were plants, but they faced a rather sterile habitat with hardly any ready nutrients. However, the migration of plants and animals to land was paralleled by that of fungi. There developed a mutual symbiotic relationship between chlorophyllous plants and fungi and this helped in the colonisation of land. The latter have a big 'arsenal' of enzymes which enabled them to perform many functions. Thus fungi played a unique role in the absorption of inorganic and other essential nutrients. Therefore, symbiotic mycorrhizae were a critical input in the establishment of land flora. Fungi, in turn, benefited from the organic carbon from plant roots. Healthy plant growth was well-established by the Cambrian era, as were the food pyramids. There is clear evidence that during the Devonian period, early plants and metazoans expanded rapidly as a precursor to largescale occupancy of land in the Carboniferous.

Whether the vari ous subgroups of plants and animals emerged from water at a single or at more than one location, is indeed a moot question. However, there is no doubt that many of the transitions have vanished without leaving too many direct or indirect fossil records. The convincing evidence on terrestrial organisms comes from the sediments of the Ordovician era. Moreover, the evidence as available today indicates that atmospheric parameters that affected temperature and uv flux occurred during the late Proterozoic era and set the stage for occupancy of land.

It is well known that the topography of land has been changing and there are many gaps in our knowledge of plants and animals. The invasion of land has not been a single event. There must have been many such events coupled with major constraints in the development of terrestrial soil. The other constraint to our understanding of the transition is that there are hardly any fossil records of algae, bryophytes and early vascular plants.

This book is original in its approach and is timely. The knowledge gained about the transition of organisms from aquatic to terrestrial life would not only be useful for the sake of new knowledge but also help in the colonising of some inhospitable habitats on the earth. Biologists, evolutionists and ecologists will find it very instructive.

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