is evidence that placental mammals and marsupials split apart in their evolution more than 160 MYA Before discovering 1. sinensis scientists had estimated when this split occurred They did this by comparing marsupial DNA and placental mammal DNA based on the molecular clock hypothesis The molecular clock hypothesis says that the order of proteins and the DNA sequences that they make up evolve at a constant rate that is unchanged over time and between organisms Scientists compare DNA samples from two or more organisms Based on the number of differences they can estimate how long ago the organisms shared a common ancestor The comparison between marsupial and placental mammal DNA suggested that the split had occurred approximately 160 MYA but there were no fossils old enough to support this date The previous oldest known placental mammal fossil was from 125 MYA The discovery of J. sinensis helped confirm when these two mammalian branches diverged or split into two distinct groups Understanding how this took place requires a basic understanding of evolution EVOLUTION When scientists speak of biological evolution they are talking about the physical changes that take place in animal populations and species over time Evolution doesn't mean a group of
animals is getting better but it doesn't mean the group is getting worse either It simply means a plant or animal species is changing often in response to its environment One of the first people to try to explain how species change over time was French naturalist Jean Baptiste Lamarck In 1809 he theorized that animals could gain a trait over a lifetime and pass it down to their offspring He used giraffes as an example explaining that giraffes reach up to feed on tree leaves stretching their necks and front legs Their necks and front legs got longer and these traits were passed down to their offspring It didn't take long for his fellow FOSSILIZATION Fossilization is a process through which a plant or animal or an animal's tracks is preserved after it dies Certain conditions must be met for fossilization to take place although it can occur in several different ways Sometimes water buries a plant or animal in sediment such as sand silt or gravel That is when the most common form of fossilization called petrification can occur Buried soft tissues decay but the bone is left behind Water carrying minerals seeps into the bones and deposits the minerals which crystallize Then the bones harden along with the sedimentary rock In another process called an external mold when an organism completely dissolves it can leave a space shaped just like itself in the sediment When this shape fills with another mineral a cast is formed A similar process makes what is known as an internal mold This is created when minerals fill the inside of a skull or shell and the remains then dissolve
scientists to disprove this theory since they could observe that large muscles from hard work and exercise are not passed on to offspring In 1858 Englishmen Charles Darwin and Alfred Russel Wallace independently proposed the theory of evolution through natural selection In their theory survival and reproduction were the mechanisms through which change occurred Darwin explained that individuals within a species could have a number of diverse traits These traits would make some animals faster or better able to blend in with the landscape than others The animals with beneficial traits for their environment would survive longer and perhaps because of this have more offspring Many of these offspring would inherit the beneficial traits survive reproduce and pass the traits on to their own offspring As this happened through several generations the positive traits would be present in more individuals and the less beneficial traits would slowly disappear As certain traits became more common and others died out the species evolved Darwin understood that these changes happened but he did not know exactly how It would take Gregor Mendel's discovery of heredity in 1866 to explain how these changes took place Scientists later learned that heredity is caused by genes in DNA Genes are the information encoded in DNA They pass from parent to offspring They determine traits such as what color an animal's fur is or how much muscle it can build Scientists eventually realized that
According to Darwin's theory traits such as cheetahs speed helped them survive and pass on those traits to offspring change could happen as the result of a mutation A mutation is when the structure of a gene is accidentally changed because proteins or other molecules are substituted for others during reproduction A beneficial mutation could assure better success in survival and reproduction and slowly spread through the population Bad mutations make the animal more likely to die early and less likely to reproduce Over millions of years descendants of relatives of J. sinensis evolved into human beings cats and even whales If a new trait develops throughout an entire species the species as a whole evolves Sometimes the populations of a species are divided into two or more geographically separated groups This might happen when the sea level rises and floods a land bridge Ora river might cut a new channel creating an island on which part of the species lives Groups that are separated cannot interbreed Because of this one group will change in one way while another changes in another way Eventually the two groups will be so changed that they will become different species This process is how the ancestral mammals similar to J. sinensis eventually evolved into the more than 6,000 species of modern mammals
