Saturday, March 18, 2017

Comments On The 1.6 Billion Year Old Red Algae From Central India

The Proterozoic Vindhyan sedimentary basin in Central India contains sediments ranging in age from 1.7 billion years to about 600 million years ago. Bengtson and colleagues report three dimensional preservation of cellular structures which they interpret as multicellular red algae. These fossils have been found in the Tirohan Dolomite dated to about 1. 6 billion years. Before this discovery, the earliest fossils of multicellular eukaryotes was the rhodophyte Bangiomorpha, dated to about 1.2 billion years.

The Tirohan Dolomite is exposed in the Chitrakoot region of Madhya Pradesh. The fossils occur in patches of carbonate sediment which was replaced by the calcium phosphate mineral apatite just after their deposition in a shallow marine setting. Phosphotization is often a very delicate process enablng the preservation of fragile cell structures.

Here is a picture of the cellular structures of red algae imaged by SEM (scanning electron microscope)

Source: Bengtson 2017

And another rendering of the three dimensional structure of the red algae imaged using Synchrotron-Radiation X-ray Tomographic Microscopy (SRXTM). The green objects inside the cell are interpreted to be organelles, components of eukaryotic cells which aid in different physiological functions. Prokaryotes (Bacteria) lack such organelles.

Source: Bengtson 2017

I don't want to dwell on this study too much. The paper is open access for those who want to explore further.

There are two side stories that I want to comment upon.

First. The Tirohan Dolomite and its fossil assemblage has a controversial past.

They were discovered about twenty years ago by Dr Rafat Azmi, a paleontologist working with the Wadia Institute of Himalayan Geology. He reported from the Rohtasgarh area in 1998 a rich trove of filamentous and spherical forms, and odd shaped mineral fragments. He interpreted the mineral fragments as "small shelly fossils" representing fragments of animal shells and the spherical forms as possible animal embryos. Later in 2006 he reported tubular forms which he interpreted as Cambrian animal taxa. The problem was that animals are thought to have evolved by the latest Neoproterozoic- early Cambrian (600 mya -540 mya), while the understanding then was that the Tirohan Dolomite is likely 1 billion to 1.5 billion years old. Azmi's interpretation carried two enormous implications; either a) the Tirohan Dolomite was much younger in age. This would have required a major revision of the ages of Vindhyan sediments or b) that the rocks were old (~1.5 billion years), but that animals evolved much earlier than the current fossil record indicated.

These very significant implications caught the attention of geologists and media alike. The Geological Society of India sent a team to investigate Dr. Azmi's claims. They reported that they were unable to find the fossils Dr. Azmi had claimed to have found.

 Memories of an earlier scandal in Indian palaeontology were still fresh. In the late 1980's Vishwajit Gupta of Punjab University was found guilty of fraud and plagiarism. He had been misreporting fossil discoveries from the Himalayas by using museum specimens from all over the world. He had  constructed an entirely fake narrative of Himalayan fossils and stratigraphy. Scientific journals were forced to retract his papers. The Paleontological Society of India produced a book authored by S. K Shah titled "The Himalayan Fossil Fraud".  Punjab University, disgracefully, allowed Dr. Gupta to remain in service till he retired in 2004.

Under this shadow, Azmi's fossils came under similar suspicion. Fortunately, Bengtson and colleagues in a study some years later confirmed that these fossils do exist in the Tirohan Dolomite. However, they sampled the Tirohan Dolomite at Chitrakoot and not its stratigraphic equivalent (Rohtas limestone) at Rohtasgarh where Dr. Azmi's initial claims came from. They established using absolute radiometric dating that the Tirohan Dolomite is 1.6 billion years old. And they showed that the forms, similar to those Dr. Azmi found, are not multicellular animals. The spherical forms were all likely gas bubbles. Some of the larger tubular forms were revealed in the present study as red algae. Animal evolution didn't take place that early after all. The claim of the "small shelly fossils" has not been resolved fully. Bengtson and colleagues work doesn't address them. Some other researchers though have interpreted them as non-biogenic mineral growths. The stratigraphy and broader fossil content of the Rohtas limestone from where Azmi collected his fossils firmly indicates that it is not Cambrian but Proterozoic in age. .  In this present paper, these scientists have named one of the red algal forms Rafatazmia chitrakootensis in honor of Dr. Razat Azmi.

The second comment I have is on multicellularity. These red algae are the oldest multicelluar eukaryotes found anywhere. Plants, Fungi, Protists (amoebas) are eukaryotes.  They share a common eukaryote ancestor which was unicellular. That means there was just one origin of the eukaryotic cell type. However, multicellularity has evolved many times independently in different branches of the eukaryote family.

Multicellularity comes in different flavors. In simple forms of multicellularity, organisms are made up of sheets and aggregates of cells sticking to one another. There is differentiation of somatic and reproductive cells. Communication between cells is limited. One important aspect is that all the cells are in direct contact with the environment, since in these organisms, nutrient transfer takes place by diffusion from the environment to the cell. More complex types of multicellularity require the evolution of not just cell to cell adhesion, but elaborate cell to cell communication systems and a division of labor i.e. cells specialized for different functions. Also, these organisms have a three dimensional arrangement of cells wherein only few cell types are in direct contact with the environment. Diffusion is not efficient enough to supply internal cells with all the necessary life support. Molecular conduits and tissues that facilitate bulk transport and circulation of nutrients need to evolve to build this type of multicellularity.

The figure below shows the many origins of the complex type of multicellularity (in red) in different eukaryotes branches.

Source: Andrew H Knoll 2011

Based on cell type, life is divided into two domains. The Prokaryotes (Bacteria and Archaea) have smaller simpler cells. Eukaryotes are generally larger and are made up of more complex cells. This cell type evolved by a symbiotic merger between two types of prokaryote cells. Prokaryote fossils have been found in rocks older than 3 billion years. The eukaryote fossil record begins in rocks younger than 2 billion years. The timing of the origin of eukaryotes is unclear. Estimates range from  2.5 billion to 1.5 billion years ago. These red algae fossils show that eukaryotes had already diverged into different branches by 1.6 billion years ago, which means that the unicellular ancestor of eukaryotes evolved before that. It also means that red algae took the road to multicellularity much earlier than animals.

Does complexity evolve necessarily whenever genetic potential is available or does it depend on ecologic opportunity? If the cellular machinery and the underlying genetic regulatory systems required for multicellularity evolved in the ancestors of red algae by 1.6 billion years ago, why did multicellular animals not evolve earlier as well? It could well be that there were ecologic conditions limiting the evolution of physiologically demanding creatures like animals. The end of Neoproterozoic ice-ages by about 650 million years ago and the break up of supercontinent Rodinia impacted sea water chemistry. Sea water oxygen increased to threshold levels permitting a more active life style. Increased weathering of continents brought into the oceans metals like zinc which are crucial for physiological functions. Creation of larger continental shelves and shallow water zones due to continental breakup provided varied ecologic spaces for diversification. Animal evolution was triggered in this ecological context.

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