segunda-feira, 30 de maio de 2011

Mapeamento da Mata Atlântica

30/05/2011
Agência FAPESP – A Fundação SOS Mata Atlântica e o Instituto Nacional de Pesquisas Espaciais (Inpe) divulgaram em 26 de maio os dados do Atlas dos Remanescentes Florestais da Mata Atlântica com a situação de 16 dos 17 estados em que o bioma está presente no período de 2008 a 2010.
Da área total do bioma, 1.315.460 km², foram avaliados 1.288.989 km², o que corresponde a 98%. Foram analisados os estados de Alagoas, Bahia, Ceará, Espírito Santo, Goiás, Minas Gerais, Mato Grosso do Sul, Paraíba, Pernambuco, Paraná, Rio de Janeiro, Rio Grande do Norte, Rio Grande do Sul, Santa Catarina, Sergipe e São Paulo.


Inpe e SOS Mata Atlântica divulgam dados do Atlas dos Remanescentes Florestais da Mata Atlântica com a situação de 16 estados no período de 2008 a 2010 (E.Cesar/Pesquisa FAPESP)


De acordo com os coordenadores do levantamento, dos 17 estados abrangidos total ou parcialmente no bioma Mata Atlântica, o único não avaliado foi o Piauí, cujos dados não puderam ser incluídos pela indefinição de critérios de identificação das formações florestais naturais do bioma naquele estado.
Os dados do estudo apontam desflorestamentos verificados no período de 31.195 hectares, ou 311,95 km². Desses, 30.944 hectares correspondem a desflorestamentos, 234 a supressão de vegetação de restinga e 17 a supressão de vegetação de mangue.
Entre os estados avaliados em situação mais crítica estão Minas Gerais, Bahia, Santa Catarina e Paraná, que perderam, entre 2008 e 2010, 12.467, 7.725, 3.701 e 3.248 hectares, respectivamente. A esses números, somam-se desflorestamentos de 1.864 hectares no Rio Grande do Sul, 579 em São Paulo, 320 em Goiás, 247 no Rio de Janeiro, 237 no Espírito Santo e 117 hectares em Mato Grosso do Sul.
Nos demais estados do nordeste, foi verificada supressão de vegetação nativa a partir de 2002 que totalizaram 24 hectares em Alagoas, 253 em Pernambuco, 224 em Sergipe e 188 no Ceará. Na Paraíba e no Rio Grande do Norte não foram registrados desflorestamentos ou supressão de vegetação de restinga ou de mangue, de acordo com a metodologia adotada pela pesquisa do Atlas, que considera área mínima de mapeamento de 3 hectares.
Em todos os estados foram verificadas queda na taxa média anual de desflorestamento. Em Minas Gerais, a taxa média anual caiu 43%, já que no último levantamento, referente ao período de 2005 a 2008, o total de desflorestamento foi de 32.728 hectares. Minas Gerais possuía originalmente 46% do seu território (27.235.854 hectares) coberto pelo bioma Mata Atlântica e agora restam apenas 10,04% (2.733.926 hectares).
A Bahia, apesar de ser o segundo estado do ranking, apresentou uma queda de 52% na taxa anual média de desmatamento. Passou de 24.148 hectares, no período de 2005 a 2008, para 7.725, no período de 2008 a 2010. O estado, que já teve 33% de seu território coberto por Mata Atlântica, hoje tem a incidência do bioma em apenas 9% do seu território (1.692.734 hectares de floresta nativa).
Em Santa Catarina, apesar de o desflorestamento continuar, a taxa anual caiu 79%. O estado está inserido 100% na Mata Atlântica (9.591.012 hectares) e hoje restam apenas 23%, ou 2.210.061 hectares, do bioma original.
No Paraná, a taxa anual de desmatamento diminuiu 51%, e o estado perdeu de 2008 a 2010 mais 3.248 hectares. O Paraná possuía 98% de seu território no bioma, ou 19.667.485 hectares. Atualmente, são 2.094.392 hectares cobertos com Mata Atlântica nativa, ou seja, 10,65% do território original.
Os dados e mapas podem ser acessados em mapas.sosma.org.br.

O big bang da evolução

A fotossíntese pode até parecer um processo simples, mas não é. Em sua coluna de maio, Carlos Alberto dos Santos fala das lacunas no conhecimento sobre esse fenômeno e de como entendê-lo poderia ajudar a aumentar a eficiência do uso da energia solar.
Por: Carlos Alberto dos Santos
Publicado em 27/05/2011 | Atualizado em 27/05/2011
O big bang da evolução
Estrutura da enzima fotossistema II, responsável pela quebra da molécula de água, que resulta na produção de elétrons e prótons. O surgimento dessa enzima na Terra é considerado o “big bang da evolução”. (imagem: Curtis Neveu/ CC BY-AS 3.0)
O título, extraído de textos do bioquímico britânico James Barber, refere-se ao surgimento na Terra, há aproximadamente dois bilhões de anos, de um dos componentes do processo de fotossíntese.
Se você não é especialista do ramo, provavelmente vai dizer que a clorofila é a responsável pela cor verde da vegetação. Isso é apenas uma parte da história desse fenômeno, que é responsável por todas as formas de vida em nosso planeta.
A fotossíntese continua desafiando nossa inteligência para entender muitos de seus aspectos
Expresso em termos gerais, a fotossíntese aparenta ser muito simples. Ao incidir sobre a folha de uma planta, a luz solar produz uma reação fotoquímica, tendo água e gás carbônico como reagentes e oxigênio e glicose como produtos da reação. É o início da produção de biomassa.
Mas bastam umas poucas perguntas sobre detalhes do processo e logo se descobre sua complexidade. Não é por nada que mais de três séculos depois de sua descoberta, a fotossíntese continua desafiando nossa inteligência para entender muitos de seus aspectos.
No início do mês, a prestigiosa revista Science publicou artigo assinado por 18 pesquisadores de famosas universidades de vários países com o único objetivo de comparar cálculos de eficiência na fotossíntese e nos sistemas fotovoltaicos, uma tarefa tão difícil quanto necessária para o desenvolvimento tecnológico da energia solar.
Energia solar
Usina solar PS10, na Espanha. Pesquisadores tentam comparar a eficiência na fotossíntese e nos sistemas fotovoltaicos, uma tarefa difícil, porém necessária, para o desenvolvimento tecnológico da energia solar. (foto: Wikimedia Commons/ afloresm – CC BY 2.0)
Uma parte da dificuldade em calcular a eficiência energética da fotossíntese reside na falta de conhecimento detalhado do processo. Um alerta contundente para algumas lacunas nesse conhecimento foi dado recentemente por Marco Sacilotti e colaboradores do Departamento de Física da Universidade Federal de Pernambuco (UFPE).
Não resta mais dúvida de que o mecanismo da fotossíntese é controlado pela mecânica quântica. No entanto, a literatura atual não apresenta elementos quantitativos ou mesmo qualitativos para justificar essa hipótese, afirmam os pesquisadores da UFPE ao propor modelos para sanar essa dificuldade.
Uma das principais deficiências dos modelos atuais é a falta de uma boa compreensão das forças que determinam o movimento de cargas elétricas positivas e negativas no interior dos sistemas fotossintetizantes.

Eficiência natural

Se por um lado o tratamento quântico da fotossíntese tem muito a evoluir – o que representa um vasto campo de trabalho para biólogos, físicos e químicos –, por outro, o cenário geral do processo já está bem estabelecido.
Não há mais questionamento na literatura sobre a natureza inicial do processo, que ocorre quando a luz solar atinge a clorofila e outros pigmentos fotossensíveis presentes nas folhas de plantas e algas.
A energia absorvida nessa interação é transferida para uma enzima estruturalmente complexa, conhecida como fotossistema II (PSII, na sigla em inglês). O surgimento dessa enzima na Terra é considerado o “big bang da evolução”.

Pesquisadores acreditam que procedimentos de engenharia genética possam contribuir para aumentar a eficiência energética de sistemas naturais
Ela é responsável pela quebra da molécula da água, que resulta na produção de elétrons e prótons por meio de um processo termodinâmico com eficiência de aproximadamente 70% na conversão da energia associada à luz solar incidente em energia armazenada nas ligações químicas dos produtos formados.
Para se ter ideia do quão alta é essa eficiência, basta compará-la com os 18% da mais eficiente célula solar disponível hoje comercialmente. Todavia, até chegar ao estágio energético utilizável, os sistemas naturais dissipam energia sob diferentes formas e, ao final, a eficiência teórica não ultrapassa 4,5%, e os melhores resultados experimentais não ultrapassam 3%.
Biólogos, físicos e químicos que trabalham na área acreditam que procedimentos de engenharia genética possam contribuir para aumentar essa eficiência. Uma possibilidade seria alargar artificialmente a faixa do espectro solar absorvido por pigmentos fotossensíveis.
É que os sistemas naturais captam apenas luz na faixa visível para realizar a fotossíntese. Então, se for possível manipular esses materiais para incluir pigmentos absorvedores de outras faixas, a eficiência poderá ser maior, na medida em que mais energia será captada para a mesma intensidade de radiação solar.

Física quântica & biologia sintética

O processo inverso da fotossíntese é a fotorrespiração, produzida pela ação de oxigênio sobre glicose, que libera energia sob a forma de água e gás carbônico. A fotorrespiração chega a consumir até 25% da energia inicialmente armazenada na fotossíntese.
Para enfrentar essa limitação, a natureza desenvolveu em algumas plantas a fotossíntese C4, na qual o gás carbônico é fixado em um ácido com quatro átomos de carbono. O resultado disso é que ao apresentar maior eficiência na fixação do gás carbônico e pequena perda de água, as plantas C4 praticamente dispensam a fotorrespiração. Já existem pesquisas em andamento na tentativa de incorporar materiais fotossintetizantes do tipo C4 em plantas nas quais inexistem esses componentes.
'Fimbristylis dichotoma'
'Fimbristylis dichotoma'. Nas plantas do gênero 'Fimbristylis', ocorre a fotossíntese C4. Nesse processo, o gás carbônico é fixado em um ácido com quatro átomos de carbono, o que resulta em maior eficiência energética. (Keisotyo/ CC BY-SA 3.0)
Outras opções consideradas pela engenharia genética encontram-se em estudo, e as mais instigantes têm a ver com o uso tecnológico dos conceitos da teoria quântica para o estabelecimento da biologia sintética.
O amadurecimento das ferramentas teóricas e experimentais da biologia e da física chegou ao ponto de aplicação das ideias lançadas no início dos anos 1940 pelo físico austríaco Erwin Schrödinger (1887-1961), que ao referir-se aos organismos multicelulares disse que sua “singular engrenagem não é de grosseira manufatura humana, mas a mais requintada obra-prima já conseguida pelas leis da mecânica quântica”.

Carlos Alberto dos Santos
Professor-visitante sênior da Universidade Federal da Integração Latino-americana
From soup to cells — the origin of life

A microbe-like cellular filament found in 3.465 billion year old rock
A microbe-like cellular filament found in 3.465 billion year old rock
Evolution encompasses a wide range of phenomena: from the emergence of major lineages, to mass extinctions, to the evolution of antibiotic resistant bacteria in hospitals today. However, within the field of evolutionary biology, the origin of life is of special interest because it addresses the fundamental question of where we (and all living things) came from.
Many lines of evidence help illuminate the origin of life: ancient fossils, radiometric dating, the phylogenetics and chemistry of modern organisms, and even experiments. However, since new evidence is constantly being discovered, hypotheses about how life originated may change or be modified. It's important to keep in mind that changes to these hypotheses are a normal part of the process of science and that they do not represent a change in the basis of evolutionary theory.
Here, you can learn about important hypotheses regarding when, where and how life originated and find out how scientists study an event that occurred so long ago.


print print
When did life originate?
Evidence suggests that life first evolved around 3.5 billion years ago. This evidence takes the form of microfossils (fossils too small to be seen without the aid of a microscope) and ancient rock structures in South Africa and Australia called stromatolites. Stromatolites are produced by microbes (mainly photosynthesizing cyanobacteria) that form thin microbial films which trap mud; over time, layers of these mud/microbe mats can build up into a layered rock structure — the stromatolite.
Stromatolites are still produced by microbes today. These modern stromatolites are remarkably similar to the ancient stromatolites which provide evidence of some of the earliest life on Earth. Modern and ancient stromatolites have similar shapes and, when seen in cross section, both show the same fine layering produced by thin bacterial sheets. Microfossils of ancient cyanobacteria can sometimes be identified within these layers.
stromatolites at Shark Bay close up of a stromatolite at Shark Bay
Modern stromatolites in Shark Bay, Australia


cross section of fossil stromatolites cross section of fossil stromatolites
Cross sections of 1.8 billion year old fossil stromatolites at Great Slave Lake, Canada

Where did life originate?

Hydrothermal vent photo
A hydrothermal vent at the bottom of the ocean
Scientists are exploring several possible locations for the origin of life, including tide pools and hot springs. However, recently some scientists have narrowed in on the hypothesis that life originated near a deep sea hydrothermal vent. The chemicals found in these vents and the energy they provide could have fueled many of the chemical reactions necessary for the evolution of life. Furthermore, using the DNA sequences of modern organisms, biologists have tentatively traced the most recent common ancestor of all life to an aquatic microorganism that lived in extremely high temperatures — a likely candidate for a hydrothermal vent inhabitant! Although several lines of evidence are consistent with the hypothesis that life began near deep sea vents, it is far from certain: the investigation continues and may eventually point towards a different site for the origin of life.

How did life originate?
Living things (even ancient organisms like bacteria) are enormously complex. However, all this complexity did not leap fully-formed from the primordial soup. Instead life almost certainly originated in a series of small steps, each building upon the complexity that evolved previously:

  1. Simple organic molecules were formed.
    Simple organic molecules, similar to the nucleotide shown below, are the building blocks of life and must have been involved in its origin. Experiments suggest that organic molecules could have been synthesized in the atmosphere of early Earth and rained down into the oceans. RNA and DNA molecules — the genetic material for all life — are just long chains of simple nucleotides.
    a nucleotide, composed of carbon, hydrogen, nitrogen, oxygen and phosphorus atoms
  2. Replicating molecules evolved and began to undergo natural selection.
    All living things reproduce, copying their genetic material and passing it on to their offspring. Thus, the ability to copy the molecules that encode genetic information is a key step in the origin of life — without it, life could not exist. This ability probably first evolved in the form of an RNA self-replicator — an RNA molecule that could copy itself.
    a chain of nucleotides forms an RNA molecule
    Many biologists hypothesize that this step led to an "RNA world" in which RNA did many jobs, storing genetic information, copying itself, and performing basic metabolic functions. Today, these jobs are performed by many different sorts of molecules (DNA, RNA, and proteins, mostly), but in the RNA world, RNA did it all. Self-replication opened the door for natural selection. Once a self-replicating molecule formed, some variants of these early replicators would have done a better job of copying themselves than others, producing more "offspring." These super-replicators would have become more common — that is, until one of them was accidentally built in a way that allowed it to be a super-super-replicator — and then, that variant would take over. Through this process of continuous natural selection, small changes in replicating molecules eventually accumulated until a stable, efficient replicating system evolved.
  3. Replicating molecules became enclosed within a cell membrane.
    The evolution of a membrane surrounding the genetic material provided two huge advantages: the products of the genetic material could be kept close by and the internal environment of this proto-cell could be different than the external environment. Cell membranes must have been so advantageous that these encased replicators quickly out-competed "naked" replicators. This breakthrough would have given rise to an organism much like a modern bacterium.
    genetic material enclosed in membranes
    Cell membranes enclose the genetic material.

  4. Some cells began to evolve modern metabolic processes and out-competed those with older forms of metabolism.
    Up until this point, life had probably relied on RNA for most jobs (as described in Step 2 above). But everything changed when some cell or group of cells evolved to use different types of molecules for different functions: DNA (which is more stable than RNA) became the genetic material, proteins (which are often more efficient promoters of chemical reactions than RNA) became responsible for basic metabolic reactions in the cell, and RNA was demoted to the role of messenger, carrying information from the DNA to protein-building centers in the cell. Cells incorporating these innovations would have easily out-competed "old-fashioned" cells with RNA-based metabolisms, hailing the end of the RNA world.
    DNA contains instructions.  RNA copies DNA.  Proteins are made from copies instructions.
  5. Multicellularity evolved.
    As early as two billion years ago, some cells stopped going their separate ways after replicating and evolved specialized functions. They gave rise to Earth's first lineage of multicellular organisms, such as the 1.2 billion year old fossilized red algae in the photo below.
    Bangiomorpha pubescens Bangiomorpha pubescens These fossils of Bangiomorpha pubescens are 1.2 billion years old. Toward the lower end of the fossil on the left there are cells differentiated for attaching to a substrate. If you look closely at the upper part of the fossil on the right, you can see longitudinal division that has divided disc-shaped cells into a number of radially arranged wedge-shaped cells, as we would see in a modern bangiophyte red alga.


Studying the origin of life
The origin of life might seem like the ultimate cold case: no one was there to observe it and much of the relevant evidence has been lost in the intervening 3.5 billion years or so. Nonetheless, many separate lines of evidence do shed light on this event, and as biologists continue to investigate these data, they are slowly piecing together a picture of how life originated. Major lines of evidence include DNA, biochemistry, and experiments.
Origins and DNA evidence
Biologists use the DNA sequences of modern organisms to reconstruct the tree of life and to figure out the likely characteristics of the most recent common ancestor of all living things — the "trunk" of the tree of life. In fact, according to some hypotheses, this "most recent common ancestor" may actually be a set of organisms that lived at the same time and were able to swap genes easily. In either case, reconstructing the early branches on the tree of life tells us that this ancestor (or set of ancestors) probably used DNA as its genetic material and performed complex chemical reactions. But what came before it? We know that this last common ancestor must have had ancestors of its own - a long line of forebears forming the root of the tree of life - but to learn about them, we must turn to other lines of evidence.
The 3 domains that include all living things, the most recent common ancestor of all living things, and forebear lineages from before that most recent common ancestor 

print print
Origins and biochemical evidence
By studying the basic biochemistry shared by many organisms, we can begin to piece together how biochemical systems evolved near the root of the tree of life. However, up until the early 1980s, biologists were stumped by a "chicken and egg" problem: in all modern organisms, nucleic acids (DNA and RNA) are necessary to build proteins, and proteins are necessary to build nucleic acids - so which came first, the nucleic acid or the protein? This problem was solved when a new property of RNA was discovered: some kinds of RNA can catalyze chemical reactions — and that means that RNA can both store genetic information and cause the chemical reactions necessary to copy itself. This breakthrough tentatively solved the chicken and egg problem: nucleic acids (and specifically, RNA) came first — and later on, life switched to DNA-based inheritance.
Another important line of biochemical evidence comes in the form of surprisingly common molecules. As you might expect, many of the chemical reactions occurring in your own cells, in the cells of a fungus, and in a bacterial cell are quite different from one another; however, many of them (such as those that release energy to power cellular work) are exactly the same and rely on the exact same molecules. Because these molecules are widespread and are critically important to all life, they are thought to have arisen very early in the history of life and have been nicknamed "molecular fossils." ATP, adenosine triphosphate (shown below), is one such molecule; it is essential for powering cellular processes and is used by all modern life. Studying ATP and other molecular fossils, has revealed a surprising commonality: many molecular fossils are closely related to nucleic acids, as shown below.
adenine nucleotide and ATP
The discoveries of catalytic RNA and of molecular fossils closely related to nucleic acids suggest that nucleic acids (and specifically, RNA) were crucial to Earth's first life. These observations support the RNA world hypothesis, that early life used RNA for basic cellular processes (instead of the mix of proteins, RNA, and DNA used by modern

Origins and experimental evidence
Experiments can help scientists figure out how the molecules involved in the RNA world arose. These experiments serve as "proofs of concept" for hypotheses about steps in the origin of life — in other words, if a particular chemical reaction happens in a modern lab under conditions similar to those on early Earth, the same reaction could have happened on early Earth and could have played a role in the origin of life. The 1953 Miller-Urey experiment, for example, simulated early Earth's atmosphere with nothing more than water, hydrogen, ammonia, and methane and an electrical charge standing in for lightning, and produced complex organic compounds like amino acids. Now, scientists have learned more about the environmental and atmospheric conditions on early Earth and no longer think that the conditions used by Miller and Urey were quite right. However, since Miller and Urey, many scientists have performed experiments using more accurate environmental conditions and exploring alternate scenarios for these reactions. These experiments yielded similar results - complex molecules could have formed in the conditions on early Earth.
This experimental approach can also help scientists study the functioning of the RNA world itself. For example, origins biochemist, Andy Ellington, hypothesizes that in the early RNA world, RNA copied itself, not by matching individual units of the molecules (as in modern DNA), but by matching short strings of units — it's a bit like assembling a house from prefabricated walls instead of brick by brick. He is studying this hypothesis by performing experiments to search for molecules that copy themselves like this and to study how they evolve.
Two views of RNA replication in the early RNA world 

A knotty problem...
All the evidence gathered thus far has revealed a great deal about the origin of life, but there is still much to learn. Because of the enormous length of time and the tremendous change that has occurred since then, much of the evidence relevant to origins has been lost and we may never know certain details. Nevertheless, many of the gaps in our knowledge (gaps that seemed unbridgeable just 20 years ago) have been filled in recent years, and continuing research and new technologies hold the promise of more insights. As Ellington puts it, "Origins is a huge knotty problem — but that doesn't mean it's an insoluble one."
The Origins Problem: knotty, but not insoluble.
Fonte: http://evolution.berkeley.edu

sexta-feira, 27 de maio de 2011

Cambrian super-predators grew large in arms race

Metre-long anomalocaridids survived millions of years later than was thought.
beastThe giant Ordovician anomalocaridids were probably similar to this Cambrian Laggania.Esben Horn
 
The Cambrian Period's most ferocious predator clung to life for 30 million years longer than was previously thought. Fossils from Morocco show that sea creatures known as anomalocaridids survived long after they had been understood to have gone extinct, and their bodies grew to lengths in excess of one metre.
"Anomalocaridids are always depicted as these fierce horrible predators, ripping up things and tearing them apart — and no doubt some of them were," says Peter Van Roy, a palaeontologist at Yale University in New Haven Connecticut, who describes the findings today in Nature1, with his colleague Derek Briggs. The creatures were thought to have died out by the end of the Cambrian Period, about 500 million years ago, but "this discovery shows that anomalocaridids persisted for a lot longer and were still very successful predators at the top of the food chain".

Although the latest fossils were unearthed in North Africa and the imposing invertebrates are known to have prowled oceans worldwide, anomalocaridids are inextricably linked with the Burgess Shale, a rock formation in western Canada that contains the fossils that helped to define the Cambrian explosion — a time when strange-looking marine animals proliferated.
Anomalocaridids are bizarre even by Cambrian standards. From the late nineteenth century onwards, fossils of various body parts from the creatures were discovered separately, and attributed to ancient relatives of shrimp, sea cucumbers, jellyfish and arthropods. Only in 1985 did Briggs and a colleague realize that these bits and pieces belonged to a single kind of animal, with two tentacle-like appendages at its head, a flat, segmented body and a mouth shaped like a pineapple ring with teeth projecting towards the centre. They named it Anomalocaris2.

Fossil discoveries have since revealed that anomalocaridids came in diverse shapes and sizes — from Hurdia victoria, with its triangular carapace, to Schinderhannes bartelsi, with its long, pointed tail — and lived in the areas that are now Europe, the United States, Australia and China. But ancient relatives of sea scorpions and nautiluses that emerged in the Ordovician Period (490 million–440 million years ago) were suspected to have out-competed the anomalocaridids, causing them to die out, says Van Roy.
In 2008, however, an amateur collector, Mohammed Ben Said Ben Moula, discovered specimens that looked like anomalocaridids, Van Roy says. But it wasn't until 2009, when the researchers took a trip to the Fezouata rock formation in southeast Morocco, that they realized just what Ben Moula had discovered.
The rocks were from the early Ordovician Period, about 488 million–472 million years old — much younger than any in which such fossils had previously been found. "It was quite an indescribable moment when you're putting these things together and suddenly you realize this is an anomalocaridid," says Van Roy.

Beast of the deep

One formation contained the splayed-out, headless body of a beast more than a metre long — nearly twice as large as any of its Cambrian brethren — and more than three times the size of even the largest of the other fossilized species from the same rocks.
None of those other hard-bodied fossils from Fezouata showed the tell-tale marks of an attack from an anomalocaridid's strange mouth, so Van Roy thinks that the one-metre monsters probably hunted soft-bodied invertebrates. Their victims may have been ensnared by the anomalocaridid's giant appendages and then delivered to its mouth.
Jan Bergström, a palaeontologist at the Swedish Museum of Natural History in Stockholm, says that the presence of anomalocaridids in the Ordovician is "surprising news".
Allison Daley, a palaeontologist at the Natural History Museum in London, says that it is difficult to know what caused the extinction of anomalocaridids, but she is still willing to pin the blame on competition from other marine predators, albeit later ones than had been thought — such as cephalopods that emerged during the Ordovician.
The giant bodies of the Fezouata anomalocaridids, Daley speculates, could be the result of an ecological arms race with the emerging predators — one that the Cambrian-era monsters lost. "I doubt there was a period of happy coexistence between the anomalocaridids and newly evolving Ordovician predators," she says. 
  • References

    1. Van Roy, P. & Briggs, D. E. G. Nature 473, 510-513 (2011). | Article |
    2. Whittington, H. B. & Briggs, D. E. G. Phil. Trans. R. Soc. Lond. B 309, 569-609 (1985). | Article | ISI |

Fossil Evidence on Origin of the Mammalian Brain

  1. Timothy B. Rowe1,*,
  2. Thomas E. Macrini2, and
  3. Zhe-Xi Luo3
+ Author Affiliations
  1. 1Jackson School of Geosciences, University of Texas, C1100, Austin, TX 78712, USA.
  2. 2Department of Biological Sciences, St. Mary’s University, San Antonio, TX 78228, USA.
  3. 3Section of Vertebrate Paleontology, Carnegie Museum of Natural History, Pittsburgh, PA 15213, USA.
  1. *To whom correspondence should be addressed. E-mail: rowe@mail.utexas.edu

Abstract

Many hypotheses have been postulated regarding the early evolution of the mammalian brain. Here, x-ray tomography of the Early Jurassic mammaliaforms Morganucodon and Hadrocodium sheds light on this history. We found that relative brain size expanded to mammalian levels, with enlarged olfactory bulbs, neocortex, olfactory (pyriform) cortex, and cerebellum, in two evolutionary pulses. The initial pulse was probably driven by increased resolution in olfaction and improvements in tactile sensitivity (from body hair) and neuromuscular coordination. A second pulse of olfactory enhancement then enlarged the brain to mammalian levels. The origin of crown Mammalia saw a third pulse of olfactory enhancement, with ossified ethmoid turbinals supporting an expansive olfactory epithelium in the nasal cavity, allowing full expression of a huge odorant receptor genome.
Brain size and sensory faculties diversified dramatically as mammals evolved to fill an immense variety of ecological niches, and much attention has been devoted to reconstructing the organization and origin of the ancestral mammalian brain. Among living taxa, mammals have the largest brains relative to body size and are unique in possessing the neocortex (isocortex) (Fig. 1). Accordingly, research has focused on origin of the neocortex (15) and evolutionary increases in brain size [measured as a function of body mass, or “encephalization quotient” (EQ) (6, 7)].
Fig. 1
HRXCT images of (A and B) Monodelphis, (C and D) Hadrocodium, and (E and F) Morganucodon, in lateral and dorsal views, with bone cutaway [(A) and (B)] and rendered translucent [(C) to (F)] to show endocasts. Cb, cerebellum; Et, endoturbinals 1 to 5; Fan, annular fissure; Iam, internal acoustic meatus; II, optic nerve; Mt, maxilloturbinal; Ncx, neocortex; Nt, nasoturbinal; Ob, olfactory bulb; Ocx, olfactory (pyriform) cortex; Pfl, paraflocculus; Rf, rhinal fissure; and Sv, venous sinus.
Mammalia arose in or before the Early Jurassic [~200 million yeas ago (Ma)] (811). The oldest fossils are mostly tiny isolated jaws and teeth, and until now the rare skulls offered little detail on early brain evolution because internal access required destructive sampling. Comparative and developmental anatomy of living mammals has been our chief source of information. Such studies postulated numerous drivers for increased encephalization and origin of the neocortex, including innovations in hearing, feeding, taste, olfaction, miniaturization, parental care, endothermy, elevated metabolism, and nocturnality (17). Although deeply informative, few details have emerged on timing or sequences of historical events.
Here, we ask what sequence of evolutionary events culminated in the origin of the mammalian brain, and how was the brain in the ancestral mammal different from its closest extinct relatives? For this study, we used high-resolution x-ray computed tomography (12) to nondestructively scan tiny fossil skulls of two basal mammaliaforms from the Early Jurassic of China (Fig. 1), Morganucodon oehleri (911) and Hadrocodium wui (13). As a test of postulated neurobiological drivers, we digitally extracted casts of their endocranial cavities (endocasts), which closely approximate the size and shape of the brain, and compared them with endocasts of seven more primitive fossils and 27 crown mammals (14). The scans yielded digital measurements and anatomical details (Fig. 2) that offer a nuanced sequence of historical events in early brain evolution.
Fig. 2
Digital endocasts of (A to D) Morganucodon and (E to H) Hadrocodium in dorsal [(A) and (E)], ventral [(B) and (F)], right lateral [(C) and (G)] and left lateral [(D) and (H)] views. Cb, cerebellum; Fr1 and Fr2, postmortem fractures displacing parts of endocast; Fan, annular fissure; Hyp, hypophysis; Iam, internal acoustic meatus; II, optic nerve; Ncx, neocortex; Ob, olfactory bulb; Ocx, olfactory (pyriform) cortex; Pfl, paraflocculus; Sss, superior sagittal sinus; and V, trigeminal nerve.
The mammalian lineage (Synapsida) diverged from other tetrapods in the Carboniferous (~300 Ma) (15). The braincase initially lacked fully ossified walls and floor; hence, little is known of early brain form, and EQ estimates are imprecise. The first detailed view of the pre-mammalian brain is seen in basal Cynodontia, a clade originating in the Late Permian (~260 Ma) that includes living mammals and their proximate extinct relatives. The cynodont endocranial cavity is more fully enclosed, with EQs initially measuring from ~0.16 to 0.23 (Fig. 3) (1620). The olfactory bulbs were small (12), and the nose lacked ossified turbinals. The forebrain was narrow and featureless, the midbrain exposed dorsally, and the pineal eye persisted. The cerebellum was wider than the forebrain, and the spinal cord was narrow (12). The middle ear ossicles remained massive and attached to the lower jaw, and the cochlea occupied only a shallow bony recess (16, 21, 22). Compared with their living descendants, early cynodonts possessed low-resolution olfaction, poor vision, insensitive hearing, coarse tactile sensitivity, and unrefined motor coordination. Sensory-motor integration commanded little cerebral territory.
Fig. 3
Patterns of brain evolution in basal cynodonts and selected crown Mammalia. EQ is shown in bar chart; selected endocasts are scaled to EQ (12).
Morganucodon is the basal-most member of Mammaliaformes, a clade including mammals and their closest extinct relatives (911, 13, 15). It records a first major pulse in encephalization with an EQ of ~0.32, which is nearly 50% larger than in basal cynodonts (Fig. 3). The olfactory bulb and olfactory (pyriform) cortex are by far the regions of greatest expansion (Fig. 2). A deep annular fissure encircles the olfactory tract, marking a distinctive external division of the mammalian brain between the olfactory bulb and cortex. The cortex is inflated and wider than the cerebellum, covering the midbrain and the pineal stalk. The cerebellum is also enlarged, implying expansion of the basal nuclei, thalamus, and medulla, and the spinal cord is thicker. The brain now resembles living mammals more than basal cynodonts in shape and proportions.
Elaboration of the neocortex probably also contributed to encephalization in basal mammaliformes. Dominating the neocortex is a single primary somatosensory field (1) that maps sensation from mechanoreceptors in the skin, hair follicles, muscle spindles, and joint receptors (Fig. 4A). Its conscious component involves tactile exploration and body surface monitoring (3). Peripheral somatosensory input is mapped to the neocortex as an “animunculus” (Fig. 4A). A parallel neocortical motor map contains pyramidal neurons that give rise to the pyramidal tract (Fig. 4B), which projects via the brainstem into the spinal column to program and execute skilled movements requiring precise control of distal musculature (3, 2325).
Fig. 4
Circuitry schematic of modern opossum (Didelphis) brain showing (A) sensory input and (B) motor outputs [modified after (3)]. (C) Schematic innervation of an opossum guard hair [modified after (28)].
In living mammals, the boundary between neocortex and olfactory cortex is marked by the rhinal fissure. This structure is not visible on the endocast of Morganucodon or Hadrocodium and is faint (Fig. 1A) or invisible on endocasts in most small living mammals, although observable on the brain itself (6, 22, 26). However, another basal mammaliaform, Castorocauda lutrasimilis (27), preserves integumentary evidence suggesting that the neocortex was well developed. Castorocauda is a Middle Jurassic (~165 Ma) docodont (27), a clade first appearing in the Late Triassic and closely related to Morganucodon (911). Castorocauda is known from a flattened skeleton that preserves the oldest evidence of a thick pelt that covered the body. Both guard hairs and an underfur of vellus hairs left carbonized residues and physical impressions as thin grooves and traces.
Body hair develops as migrating neural crest cells induce patterns of tiny placodes that mature into hair follicles equipped with mechanoreceptors (25). These include lanceolate endings (velocity detectors excited by hair deflection), Ruffini receptors (tension receptors activated as hair is bent), and Merkel cells (slowly adapting sensors) (Fig. 4C). In ontogeny, hair is first sensory, and only later does it insulate, as underfur thickens and thermoregulation matures (28). Tactile signals are transmitted to the primary somatosensory field, where their morphogenic action induces formation of the sensory and motor maps (2325). The pelt in Castorocauda, in addition to the size and shape of the endocast in Morganucodon, implies that the neocortex differentiated early in mammaliaform history.
Increased sensitivity in olfaction, and improved tactile resolution and motor coordination account for much of the first pulse in pre-mammalian encephalization. Enhanced high-frequency hearing is also implicated. The middle ear ossicles are highly reduced (but still attached to the lower jaw), and the cochlea is now prolonged into a short, curved tube (9). Comparative neuroanatomy (1) suggests that neocortical expansion also supported an enhanced visual field (Fig. 4A), but bony correlates are lacking in these fossils.
Hadrocodium is the closest known extinct relative of crown Mammalia (9, 11, 13). It marks a second encephalization pulse, with an EQ of ~0.5 that lies within the mammalian range (Fig. 3). Expanded olfactory bulbs and olfactory cortex account for most of the increase. The middle ear ossicles are now detached from the jaw and suspended beneath the cranium, a condition otherwise confined to crown Mammalia (10, 11, 13, 15). Growth of the olfactory cortex in early ontogeny of the living didelphid Monodelphis separates the auditory ossicles from their primary (and ancestral) attachment to the mandible (20, 21) to develop the same anatomical relations seen in Hadrocodium. This famous transformation evidently had little effect on hearing performance because the size and complexity of the cochlea is no different than in Morganucodon (9, 13, 22, 29). The cerebellum in Hadrocodium bulges backward, bending the occipital plate into an arch that transmitted a thick spinal cord, implying enhanced motor-sensory integration.
The origin of crown Mammalia marks a third pulse of olfactory elaboration, as the ethmoid turbinals ossify to form both the cribriform plate and a rigid scaffold in the nasal cavity for epithelium containing the odorant receptor (OR) neurons (10, 15). Activation of OR genes induces olfactory epithelial growth, in turn inducing turbinal growth and ossification (30). Ossified turbinals afford a 10-fold (or more) increase in olfactory epithelial surface within the nasal cavity. The maxilloturbinal also ossifies at this same time, affecting a sevenfold (or more) increase in respiratory epithelial surface (30). It functions in water balance, and its appearance in Mammalia ancestrally may reflect elevated metabolism.
Our data suggest that in basal mammaliaforms, a first pulse of encephalization was driven by increasing resolution in olfaction and tactile sensitivity and enhanced neuromuscular coordination. With a pelt, basal mammaliaforms were probably also endothermic, and the ontogeny of thermoregulation implies parental care (28). Endothermy may have been a consequence of encephalization because a large brain is metabolically expensive to maintain (5). However, metabolism is under hormonal regulation that does not command large cerebral regions, and thus did not itself drive encephalization (3). Hadrocodium records a second pulse of encephalization, probably also driven principally by olfaction.
The ancestral species of Mammalia amplified these inheritances in a third pulse of olfactory elaboration because its ossified ethmoid complex allowed full expression of its huge OR genome, which is an order of magnitude larger than in most other vertebrates (31). Only much later did acute visual and auditory systems evolve among mammals (29). In some descendents, the olfactory system was further elaborated, whereas in others it was reduced and supplanted by alternate sensory modalities, such as electroreception and sonar. But at its start, the brain in the ancestral mammal differed from even its closest extinct relatives specifically in its degree of high-resolution olfaction, as it exploited a world of information dominated to an unprecedented degree by odors and scents.

Supporting Online Material

Materials and Methods
Figs. S1 to S4
Tables S1 to S3
References
  • Received for publication 20 January 2011.
  • Accepted for publication 4 April 2011.

References and Notes

  1. Materials and methods are available as supporting material on Science Online.
  2. Acknowledgments: This research was funded by NSF DEB 0309369 (T.E.M. and T.R.), NSF EAR-0948842 (T.R.), AToL 0531767 (T.R.), the University of Texas Jackson School of Geosciences (T.R. and T.E.M.), and funded by NSF DEB 0316558 and EF0129959, NSF of China, Humboldt Foundation (Germany), and NGS to Z.-X.L. Endocasts and computed tomography imagery are online at www.DigiMorph.org
    Fonte: http://www.sciencemag.org/content/332/6032/955.full.html#related