The saga of how ‘plants’ colonized the land and helped to make it a suitable habitat for themselves and other life forms – including animals – is oft-told and is a plausible enough story. Whilst that’s a tale with a broad sweep that extends over hundreds of millions of years, from the Ordovician (starting c. 490 millions of years ago) and continues to the present day, it’s easy to overlook the myriad small steps along that journey that paved the way for subsequent advances.
For example, merely clinging precariously to an otherwise barren rock is not enough to establish more than a tenuous foothold on terra firma. More important is how roots and root-like structures developed and interacted with fledgling soils to establish more substantial foundations that supported – quite literally (and maybe even encouraged…?) – development of more extensive high-rise carpets of greenery.
Adding to our appreciation of the undervalued role of underground plant organs in contributing to the greening of planet Earth is work by Jinzhuang Xue et al. Examining ancient soils – pal(a)eosols – from Yunnan (China), the Sino-Anglo-Canadian-based team have unearthed the extent to which such ancient substrates were infiltrated by extensive and complex ‘network-like’ structures. These features are inferred to be rhizomes of Drepanophycus, an extinct member of the lycopsid group of land-plants, and attest to the degree to which such soils and plant structures were intimately enmeshed and intermingled.
This intertwined soil-rhizomorph association is likely to have had dual benefits; on the one hand helping to stabilise the soil (and prevent its loss by erosion from wind and water), and on the other, providing anchorage and mechanical support to the plants – which would ultimately lead to the development of taller plants in the fullness of evolutionary time.
From Drepanophycus and related terra-forming club mosses (as some modern-day versions of the lycopsids are called) to true mosses (and other bryophytes) now, and an even more fundamental role of plants in the development of terrestrial life forms.
Although the development of an oxygen-enriched atmosphere is viewed as crucial to development of complex and multicellular life forms, quite how levels of atmospheric oxygen approaching modern-day values – of c. 21 % by volume of the atmosphere – came about has been a long-standing puzzle of Earth’s evolution. Oxygen-producing photosynthesis by cyanobacteria has been widely implicated in the so-called Great Oxygenation (or Oxidation) Event of approx. 2.4 billions of years ago (GOE), which provided a major increase in ancient atmospheric O2 levels.
But, that only achieved values up to a maximum of one-tenth of the current day’s concentration of O2. What generated the more substantial rises to near-modern day values approx. 420–400 millions of years ago? Work by Timothy Lenton et al. suggests that is thanks to the photosynthetic activities of some of the earliest colonizers of the land, multicellular, eukaryotic plants such as mosses and other bryophytes. Now that is rather humbling, and another reason to thank a green plant.
Xue, J., Deng, Z., Huang, P., Huang, K., Benton, M., Cui, Y., Wang, D., Liu, J., Shen, B., Basinger, J., & Hao, S. (2016). Belowground rhizomes in paleosols: The hidden half of an Early Devonian vascular plant Proceedings of the National Academy of Sciences, 113 (34), 9451-9456 DOI: 10.1073/pnas.1605051113
Hedges, S., Blair, J., Venturi, M., & Shoe, J. (2004). A molecular timescale of eukaryote evolution and the rise of complex multicellular life BMC Evolutionary Biology, 4 (1) DOI: 10.1186/1471-2148-4-2
Stamati, K., Mudera, V., & Cheema, U. (2011). Evolution of oxygen utilization in multicellular organisms and implications for cell signalling in tissue engineering Journal of Tissue Engineering, 2 (1) DOI: 10.1177/2041731411432365
Sessions, A., Doughty, D., Welander, P., Summons, R., & Newman, D. (2009). The Continuing Puzzle of the Great Oxidation Event Current Biology, 19 (14) DOI: 10.1016/j.cub.2009.05.054
Schirrmeister, B., Gugger, M., & Donoghue, P. (2015). Cyanobacteria and the Great Oxidation Event: evidence from genes and fossils Palaeontology, 58 (5), 769-785 DOI: 10.1111/pala.12178
Lenton, T., Dahl, T., Daines, S., Mills, B., Ozaki, K., Saltzman, M., & Porada, P. (2016). Earliest land plants created modern levels of atmospheric oxygen Proceedings of the National Academy of Sciences, 113 (35), 9704-9709 DOI: 10.1073/pnas.1604787113