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This post updated at 10:15 a.m. ET.

A Ukrainian government spokesman says one of its warplanes was shot down in the country's east by a Russian air force jet, as the U.S. and Europe stepped up sanctions on Moscow over its support of separatist rebels.

Ukraine says a ground-attack Su-25 was downed by an air-to-air missile Wednesday evening over the eastern region of Luhansk. The pilot reportedly safely ejected before the plane crashed. Vitaly Churkin, Russia's ambassador to the United Nations, denied that his country was responsible for the downed plane.

Reuters notes that Kiev's comment on the incident is "the strongest Ukrainian allegation to date of direct Russian military involvement in the conflict."

Separately, another Ukrainian Su-25 was hit by a rebel missile, but suffered only slight damage and landed safely, according to Ukrainian officials.

In another incident last week, Ukraine said one of its AN-26 transports was shot down by a ground or air-launched missile fired from Russia. Two of the eight crew members were killed.

According to the BBC:

"[Pro-Kiev] activists have pointed to videos which appear to show Grad multiple rocket launchers being fired from Russian soil in the direction of Ukraine.

"Nato says that Russian troop numbers on the border have increased again to about 12,000."

Australia became the first country in the world to repeal a carbon tax on the nation's worst greenhouse gas polluters, as Prime Minister Tony Abbott made good on a campaign promise to get rid of the unpopular law.

The Senate voted 39 to 32 to eliminate the tax enacted by the previous center-left government two years ago. The law imposed the equivalent of a $22.60 tax per metric ton of carbon dioxide emissions on about 350 of the nation's worst polluters.

"Today, the tax that you voted to get rid of is finally gone: a useless, destructive tax which damaged jobs, which hurt families' cost of living and which didn't actually help the environment," Abbott told reporters in Canberra.

Abbott's government came to power on a promise to eliminate the tax, "assuring voters that removing it would reduce household electricity bills. He plans to replace the measure with a taxpayer-financed AU$2.55 billion fund to pay industry incentives to use cleaner energy," according to The Associated Press.

However, Abbott and Environment Minister Greg Hunt have repeatedly refused to rule out a price on carbon in the future.

The Sydney Morning Herald says:

"Mr Abbott said he did not accept that with the carbon price now abolished, and legislation needed for Direct Action yet to pass the Senate, his government was leaving Australia without a mechanism to reduce greenhouse gas emissions.

"'We are a government which absolutely appreciates that we have only got one planet and we should pass it on to our children and grandchildren in at least as good shape as we found it,' he said.

"'So we are a conservationist government and we will do what we think is the sensible thing to try to bring emissions down.'"

The menus of millennia past can be tough to crack, especially when it comes to fruits and vegetables. For archaeologists studying a prehistoric site in Sudan, dental plaque provided a hint.

"When you eat, you get this kind of film of dental plaque over your teeth," says Karen Hardy, an archaeologist with the Catalan Institute for Research and Advanced Studies at the Universitat Autnoma de Barcelona.

"If you don't clean it off, it mixes up with bits of food and it gets stuck in this area below the gum," she says. "It can calcify within about two weeks, and once it's calcified it's very hard."

That plaque is so hard that it lasts thousands of years. And since Prehistoric folk were not known for their flossing habits, the plaque that survived them can serve as a kind of scrapbook for what they ate and breathed.

Hardy and her colleagues were studying skeletons from Al Khiday in Central Sudan, a burial site that was used between around 2,000 and 9,000 years ago, since before the advent of farming in the area.

Using a few isotope and chemical analysis techniques, Hardy says they found "all sorts of different things" in the teeth of 19 individuals, things like sand, dirt, pollen, plant fibers — even evidence of carbon, from breathing smoke from a fire.

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Here's an experience many of us have had: You're shopping on your smartphone. You go click on the shoes or books you want. But then, when you get to the shopping cart, you abandon ship. Visa says that's a big problem for retailers. On Wednesday, the credit card company announced it's rolling out a brand new system designed to get us to spend more money online.

One Password, Many Tokens

Visa is actually trying to fix two problems with one swipe.

First, shopping on your smartphone is great — until it gets tedious and you have to use your big thumbs to type lots of digits.

"All of the wonderful benefits that consumers get from using their mobile phone when you're out and about kind of fall apart when you get to the payment part of the process," says Jim McCarthy, Visa global head of product.

The new Visa Checkout should make it "really easy" for consumers, he says. Every Visa customer gets an account that stores their credit card number and billing address. And retailers connect with Visa to accept that account.

So when you're out and about in the digital mall, McCarthy says, "you'll just provide a username and password, and all that information will be provided securely on your behalf to the merchant for checkout." The first retailers to join the new system include Neiman Marcus and United Airlines.

Behind the scenes, there's a big security upgrade.

Visa, MasterCard and American Express all worked together on a new technology called tokenization. Under it, the 16 digits of your credit card stop running out in the wild, and stay inside the card company's server walls. The retailer just gets a randomly generated 16-digit token that can't be used anywhere else.

So if hackers break into, say, Target, the tokens they steal are no good at Amazon or Pizza Hut. "What we're trying to do is make the whole experience of someone trying to steal cards less profitable," McCarthy says.

Type Less, Spend More

Companies that sell products online — from the big box retailers to the mom-and-pop startups — have all been puzzling over how to get customers to not abandon the shopping cart.

Jamie Viggiano, head of marketing at TaskRabbit.com, says the new Visa Checkout could give a boost to online shopping. When you exchange cash or hand over a credit card, it feels like a financial transaction. A login and password don't.

The Two-Way

MasterCard, Visa Team Up To Improve Payment Security