Wednesday, December 14, 2016

3.66 Million Year Old Hominin Footprints From Laetoli Tanzania

I read this with a sense of wonderment:

New footprints from Laetoli (Tanzania) provide evidence for marked body size variation in early hominins (Open Access)

 - Fidelis T Masao, Elgidius B Ichumbaki1, Marco Cherin, Angelo Barili,Giovanni Boschian, Dawid A Iurino, Sofia Menconero, Jacopo Moggi-Cecchi, Giorgio Manzi 

Source: Fidelis T Masao et al 2016: Trackway L8 with four footprints (top). Relief map of the trackway L8 (bottom).

The tracks shown in the image above is the L8 trackway from site S, made by individual S1. Site S contains another set of tracks made by individual S2. Another site named site G, discovered some years earlier, lies about 150 meters to the north of site S. It contains tracks made by 3 individuals G1, G2 and G3. At both sites the tracks indicate that the hominins were walking in the same north- northeasterly direction. The tentative conclusion is that S1 is a large male, S2 and G2 are adult females and G1 and G3 are juveniles or smaller adult females. Based on fossils found in this stratigraphic succession it is believed that these tracks were made by individuals of the species Australopithecus afarensis.

Geological and stratigraphic reasoning lead to the conclusion that the trackways at site G and site S have been imprinted on the same tuff (volcanic ash) layer (Footprint Tuff) which contain a record of ash deposition over just a few weeks. Remarkably, this means that we could have a 3.66 million year old record of two contemporaneous trackways made by the same general population of hominins living in this area.

As the title of the paper suggests the important observation here is a marked body size variation interpreted from the morphology of the footprints. Below is a neat figure of stature estimates of hominins found from various localities in Africa ranging in age from 4 million years ago to 1 million years ago. The size range of A. afarensis overlaps that of later Homo.

 Source: Fidelis T Masao et al 2016

and an extract from the paper:

"These findings provide independent evidence for large body-size individuals among hominins as ancient as 3.66 Ma. Consequently, we may emphasise the conclusions by Grabowski et al. (2015) and Jungers et al. (2016), who reported that the body sizes of the australopithecines and of the early Homo representatives were similar, but also that certain australopithecine individuals (at least of Au. afarensis) were comparable with later Homo species, including H. erectus s. l. and H. sapiens. Thus, our results support a nonlinear evolutionary trend in hominin body size (Di Vincenzo et al., 2015; Jungers et al., 2016) and contrast with the idea that the emergence of the genus Homo and/or the first dispersal out of Africa was related to an abrupt increase in body size (McHenry and Coffing, 2000; Antón et al., 2014; Maslin et al., 2015). The identification of large-size individuals among the australopithecines – i.e. hominins commonly presumed to be small-bodied on average – shows also that the available fossil record can be misleading, resulting in an underestimate of the hominin phenotypic diversity in any given period".

.."Evidence for either marked or moderate body-size variation in Au. afarensis, based on data collected in a single site, was limited until now to the fossil assemblage from the Hadar 333 locality, dated to 3.2 Ma (with body masses ranging from 24.5 to 63.6 kg). The new estimates for the Laetoli individuals indicate an even more marked variation in body size within the same hominin population, at 3.66 Ma. Consequently, the combined records from Laetoli and Hadar suggest that large-bodied hominins existed in the African Pliocene for over 400,000 years, between 3.66 and 3.2 Ma. At the same time, these data contrast with the hypothesis of a temporal trend of body-size increase among Au. afarensis between the more ancient Laetoli and the more recent Hadar fossil samples (Lockwood et al., 2000)".

Another implication is that there was marked sexual dimorphism in Australopithecus afarensis possibly due to male to male competition. Their social structure may have been more similar to gorillas than chimpanzees or modern humans.

What a find!

Saturday, December 10, 2016

The Shared Fossil Heritage Of Gondwanaland

Lovely infographic:

"As noted by Snider-Pellegrini and Wegener, the locations of certain fossil plants and animals on present-day, widely separated continents would form definite patterns (shown by the bands of colors), if the continents are rejoined".

via USGS

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 due to a myriad of reasons resulting in mass extinction, 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.

Additional environmental disturbances may also contribute C12 to sea water. Warming of ocean water may lead to thawing of gas hydrates trapped below the sea bed. Methane released from hydrates is isotopically light and may break down and contribute C12 that eventually makes its way into carbonate. On land, a collapse of vegetation may release pulses of lighter carbon to the sea. Such a scenario would be realized in post Silurian times after the evolution of land vegetation.  In short, environmental catastrophe is linked to disturbances of the carbon cycle, and many sources may provide C12 to marine carbonate being formed at that time.

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.