Tuesday, December 6, 2016

Wormworld: Biological Transitions At The Precambrian-Cambrian Boundary

The earliest animals were worms and they had a profound impact on marine ecosystems.

The many theories and some new understanding on the always fascinating topic of early animal evolution has been summarized quite well in a paper by James Schiffbauer and colleagues.

Molecular divergence time estimates (e.g., Erwin et al., 2011;Peterson et al., 2008) suggest that the last common ancestor of all animals evolved in the Cryogenian (ca. 800 Ma; although see dos Reis et al., 2015, for caveats). The earliest interpreted stem-group animals, however, are the ca. 600 Ma Doushantuo embryo-like microfossils (Chen et al., 2014a; Yin et al., 2016), leaving a
200-m.y. interlude between the fossil and molecular records. This hiatus between the estimated origin of Metazoa and their first appearance in the fossil record highlights the growing realization that the earliest stages of animal diversification were neither truly Cambrian nor explosive—with the phylogenetic origin of animals temporally removed from their morphological and ecological diversification by a long fuse (e.g., Conway Morris, 2000; Xiao, 2014). 

In this case, the significant lag between the establishment of the developmental toolkits necessary for the origin of novelty and their later implementation and ecological success can perhaps be attributed to the uniqueness of newly developing animal ecosystems. Between the ignition of the fuse and the subsequent evolutionary boom, three major eco-environmental feedbacks (see Erwin et al., 2011) arose that helped to pave the way for the Cambrian Explosion: (1) linkages between the pelagic and benthic ecosystems; (2) expansion of ecosystem engineering; and (3) metazoan macropredation. These feedbacks are explored herein in the context of the terminal Ediacaran fossil
record of vermiform organisms. This “wormworld” biota— comprised of various tubicolous body fossils (Figs. 2A–2C), such as the cloudinids, and increasingly complex vermiform ichnofossils (Figs. 2D–2F)—critically occupied a fundamental phase shift from competition- to predation-governed marine benthic ecosystems.

What was the big change in macroscopic life habits from the Precambrian to Cambrian times? Macroscopic multicellular life of the Ediacaran was dominated by benthic sessile forms. Early Cambrian animals were mobile creatures engaged in predation, burrowing, grazing and reef building. These activities resulted in an ecosystem engineering of sorts. For example; a) grazing and burrowing activity churned up sediment and oxygenated it. b) the evolution of guts in bilaterians transferred nutrients from sea water to the sediment in the form of fecal pellets.  These life modes created new ecologic niches and opened up new potential evolutionary pathways.

..And what killed out the classic Ediacaran biota. Was is environmental changes or ecologic competition from early animals?

It is important to note that the suggested mass extinction of the Ediacara biota in the context of our wormworld model is an ecologically driven event rather than an environmentally driven cataclysm akin to more recent (Phanerozoic) mass extinctions, and thus may have been comparatively protracted—as evidenced by Ediacara holdovers in the early Cambrian (Conway Morris, 1993; Hagadorn et al., 2000; Jensen et al., 1998). Nonetheless, whereas the static synecology and comparatively passive feeding modes of the classic Ediacarans had once emplaced a boundary on evolutionary possibility, the successful expansion of innovative traits of herbivory and carnivory, and their causal ties to infaunalization, reef-building, and biomineralization, permitted a new scaling of this bounding “right wall” (sensu Knoll and Bambach, 2000) as realized by the organisms of the wormworld fauna. Over time, the evolutionary breakthroughs conveyed by these neoteric organisms, including novel strategies, behaviors, and physiologies, increased the heterogeneity of benthic ecosystems, allowed for enhanced exploitation of resources, and established insurmountable increases in ecospace that ultimately signaled the curtain call for the Ediacara-type guilds.

The question of extinction of Ediacaran biota though may be more open ended than that suggested in this paper. E F Smith and colleagues in a recent issue of Geology analyze carbon isotope signatures of a carbonate succession spanning the Precambrian-Cambrian boundary. They find the carbonate sediment have pronounced negative carbon isotope values signalling a collapse or significant decrease in primary productivity in the oceans.  What is the link between ecosystem collapse and negative carbon isotope excursion in carbonate sediment? Organic tissue preferentially incorporates C12, the lighter isotope of carbon. That means in thriving ecosystems, life is using up C12 from sea water and less of it makes its way into growing CaCO3 crystals forming carbonate sediment on the sea floor. When ecosystems collapse during extinctions there is more C12 available to get incorporated into carbonate sediment. This increase in the lighter isotope C12 is a negative excursion of del13C, the ratio of C13 to C12.

Anyways, what that means is that the decline in Ediacaran biota may have been due to both an environmental calamity as well as by longer term persistent competition by early animal activity.

And here is an infographic that summarizes the significant geological, ecological and biological events spanning the Precambrian-Cambrian transition

Source: Schiffbauer et al 2016

Open Access.

Tuesday, November 29, 2016

Human Evolution: The Paleolithic In The Indian Subcontinent

Came across this article by anthropologist Sheila Mishra on the Paleolithic of the Indian subcontinent and its significance in understanding human evolution.

The Indian Subcontinent is one of the areas occupied by hominins since Early Pleistocene times. The Lower Palaeolithic in the Indian Subcontinent is exclusively Acheulian. This Acheulian is similiar to the African Acheulian and has been labeled "Large Flake Acheulian" (LFA). The Middle Palaeolithic in the Indian Subcontinent is a poorly defined entity and the author has suggested that this phase should be considered the final phase of the Large Flake Acheulian from which it evolved. Microblade technology has recently been shown to be older than 45 Ka in the Indian subcontinent and is certainly made by modern humans as it has a continuity from this time until the bronze age. Presently, the nature of the transition from Acheulian technology to Microblade technology is not well understood as few sites have been dated to the relevant time period.

The continuity of the Lower Palaeolithic in the Indian Subcontinent is due to its ecological features. The Indian Subcontinent extends from approximately 8°-30° N which would normally encompass equatorial, tropical and temperate latitudinal zones. However, the influence of the monsoonal climate and sheltering effect of the Himalayan mountains results in a sub-tropical grassland vegetation extending both northwards and southwards of its normal distribution. Rainfall, rather than temperature, is the most important ecological variable which has a longitudinal rather than latitudinal variation. Thus, the Indian Subcontinent has a more homogenous environment than any comparable landmass and one eminently suitable for hominins. In contrast, the African climate zones are strongly latitudinal in distribution. The Indian Subcontinent during the Early and Middle Pleistocene has close connections with Sundaland. The fauna associated with Homo erectus in Java is derived from the Indian Pinjor faunas. During low sea levels the area of land exposed in the Sunda shelf is equal in size to the Indian Subcontinent. Sundaland has an important buffering effect on the Indian Subcontinent, with favourable conditions for Hominins in Sundaland coinciding with unfavourable ones in the Indian Subcontinent.

She interprets the ecology and tool record as suggesting that Homo erectus evolved in the India-Sundaland region and not in Africa. This scenario implies there was a migration of Homo erectus into Africa from Asia by 1.8 million years ago or so.  She points out that a number of African mammal species appear in the Indian Siwaliks (Himalaya foothills)  by 3-2.5 million years ago and so presumably an ancestral species (Australopithecus? early Homo?)  may have migrated out of Africa at that time. There have been recent announcements of putative 2.6 million year old stone tools from the Siwaliks, but their significance is still up for debate. And given the paucity of skeletal remains in India, her theory is going to be a hard sell.

There is  also a really good description of the geological context in which Paleolithic stone tools are found in the Indian subcontinent. They have been often described as "surface" sites but Mishra points out that they have been eroding from fluvial sediments. Volcanism, sedimentation and tectonics in the African rift valley and parts of Java lead to conditions favoring both burial and preservation and later exhumation of fossils and tools. The situation in India is different. Since Mio-Pliocene most of Peninsuslar India has been an erosive landscape with sedimentation occurring in a few fluvial systems with a depositional regime. Thick fluvial successions are rare. Preservation potential on the Indian landscape was low. The implication is that India may have had a larger population of hominins through the Pleistocene than the rarity of remains suggest.  Caves are the other context in which hominin fossils have been found in Africa, Europe and Asia. Have caves been adequately explored in India?

A very interesting article. Open Access.

Friday, November 18, 2016

Jesus n Mo: Those Furry Eskimos

They nail it every time!

Absence of furry "eskimos" is an actual argument touted against evolution! :)

Thursday, November 10, 2016

Photomicrograph: Treasure Inside A Brachiopod Shell

Couldn't help posting this picture. I am currently creating a catalog of carbonate textures and diagenetic fabrics for the geology department at Fergusson College, Pune, which I hope will be used as a teaching aid.

This photomicrograph captures the inside of a Mid Ordovician brachiopod shell. A complex cement sequence is present inside the pore space. The sequence represents passage of the sediment from depositional marine settings to later deep burial depths. During that long journey the sediment encountered fluids of different chemical make up resulting in the precipitation of different cement types.

Pure magic!

Tuesday, November 8, 2016

Evolution Is Still Misunderstood

Got this via @David_Bressan

Sigh... even Science gets it wrong here. I am assuming that the lighter colored, hairless and bipedal creatures shown in the figure are all hominins. Evolution is a branching process, that much is correct, but they bungled up the branching order.

Hominins are a group that consists of modern humans, all other extinct Homo and all members of the extinct ancestral taxa Australopithecus, Paranthropus and Ardipithecus to the exclusion of the chimpanzee.  In the branching tree starting from the primary branching node at the top, the left side branch contains some hominins and the chimpanzee. The right side branch contains other hominins. Since the diagram shows the chimpanzee splitting away from within the left side branch, it implies that the left branch hominins are more closely related to chimps than they are to the right branch hominins.

That can't be right!

The correct branching order should have been depicted like this:

Evolution is still misunderstood..