Size and multicellularity

Most species rarely exceeded 1 mm in size and were usually much smaller for the first 2,500 million years of life on Earth (Carroll 2001). The earliest reported, 3,500-million-year-old bacterial microfossils, averaged about 5 mm in diameter (Schopf 1994). Early eukaryotic microfossils (acritarchs), while significantly larger, ranging generally from about 40 to 200 mm in size, with a few larger exceptions (see Knoll 1992), for much of their first 600-800 million year history. The size of organisms grew substantially once multicellular forms evolved. In bacteria and algae with cell walls, one of the simplest ways of becoming multi-cellular was for the products of cell division to stay together to form long filaments, and indeed many early multicellular eukaryotes were millimetre-scale, linear or branched, filamentous forms (Knoll 1992).

The size and shape of life did not expand noticeably until the late Proterozoic. Millimetre-scale metazoans were present around 550 million years ago, as indicated by radially symmetric impressions and trace fossils (Knoll and Carroll 1999). The mysterious Ediacaran fauna, which comprised tubular, frond-like, radially symmetric forms, generally attained several centimetres in size, although some, such as Dickinsonia, approached 1 m, as did macroscopic algae (Carroll 2001). Organisms grew considerably larger in the Cambrian, with bilaterians up to 50 cm in size, and sponges and algae up to 5-10 cm (Briggs et al. 1994). The maximum body lengths of animals and algae (for examples, kelp) were subsequently to increase by another two orders of magnitude. The largest existing organisms - giant fungi and trees - evolved from independent small ancestors. Land plants probably evolved from charophyte green algae, and both green algae and plants evolved from a unicellular flagellate ancestor (e.g. Kendrick and Crane 1997). Fossil spores indicating the earliest evidence of plant life date from the mid Ordovician, and the oldest plant-body fossil (Cooksonia) suggests that early land plants were small and, on the basis of molecular phylogenetic analyses, are believed to be comparable in organization and life cycle to liverworts. Many of the principal groups of land plants have evolved large (> 10 m) species at some point in their history (Carroll 2001).

In summary, an increase in the mean and the maximum size of organisms has occurred during the evolution of multicellular bacteria, eukaryotes, and multicellular eukaryotes, and evolution within the algal, fungal, plant, and animal lineages.

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